Among applications that provide archiving features, there's a huge range of variation in how they workand how easy they make it to restore your work later. The fact that an application stores multiple revisions of each backed-up file does not, by itself, make it good for creating archives.
First, there's an important distinction to make: true archives versus rotating backups. In a true archivethat is, an additive incremental archiveevery version of every file you designate is saved, but identical files are never duplicated. In a rotating backup, the program creates a complete, separate copy of all your files every daybasically a non-incremental archive. Then, after a certain number of days (specified by the user), the program erases the oldest backup and adds a new one. Rotating backups, because they copy every single file each day, take longer to perform and require more storage space. If you've got room and time, there's nothing wrong with that approach, and it removes the need for a snapshot list (since all the files are there). However, because you're erasing files older than a certain date, you're restricting your restoration ability. If you keep, say, 5 days worth of rotating backups and find you need a file you deleted a week ago, you're out of luck.
A few applications offer the best of both worlds: rotating archives. Like a conventional archive, new files are added to the backup incrementally (without overwriting older versions). However, to conserve space, you can opt to erase the oldest versions of selected files at the same timefor example, all versions older than 30 days, or versions copied more than 30 sessions ago.
Warning! Not every program uses the same criteria to determine when a file should be added to an incremental archive. Some rely exclusively on modification dates, which is an error-prone method. For instance, simply changing the name of a file does not change its modification date. And some applications do not correctly set the modification date each time you save a file.
File Format, Compression, and Encryption
To oversimplify somewhat, most software employs one of two basic methods to copy files when performing a backup. One way is to copy each file in a stand-alone Finder-readable format, so that the backed-up files look exactly like the originals. Another way is to copy all the files into a single, larger file (sometimes called an archive file or a backup set). Each approach has advantages and disadvantages.
Finder-format copies can be restored without the use of a backup programjust drag and drop. Some people also feel more secure knowing they can get at their files easily even if their backup software goes south. Of course, the backed-up files generally take up exactly as much space as the originals (see just ahead for a discussion of RsyncX, which changes this equation somewhat).
Archive files can be compressed as they're stored, potentially saving a large amount of hard disk space. They can also be encrypted, so that if your backup media were lost or stolen, no one could read your files without knowing your passphrase. And unlike Finder copies, which normally take as their owner the user name of the person currently logged in, archive files can preserve original Unix ownership and permissions. Of course, you will need the backup software to restore files, and you could have a slightly higher risk of data loss due to file corruption (since all the data is stored in a single file)but most backup software has verification mechanisms to compensate for this.
Not all programs that offer compression or encryption copy data into a single archive file. A few can compress or encrypt individual files, such that they can be moved or copied (but not opened) in the Finder. You must still use the backup software to restore them to their original state.
RsyncX (based on the open-source command-line program rsync) deserves special mention here. Its unique copying method produces space-saving incremental archives that nevertheless look and act like complete Finder-readable copies. Here's how it works: When you perform your first archive backup, RsyncX makes a complete copy of the selected fileswith Unix ownership and permissions intact. When the next incremental backup runs, the program creates a new folder that appears to contain another complete copy of all your files. In reality, only modified files are copied; for files that have not changed, RsyncX uses a Unix trick to create a link to the original copy that appears in the Finder to be an ordinary file. This link functions somewhat like an alias in that it takes up virtually no space and merely points to another file. But when you copy this special link to another volume (when restoring files, say), you automatically copy the entire file. The upshot of this technique is that RsyncX comes quite close to offering the best of both worlds: Finder-readable files that require no more space than an archive file.
However, you should also be aware of another option: disk images. Some backup software, at least when backing up to a hard disk, stores files in a disk image. (Apple Backup 3 uses this technique, although its disk images are hidden inside packages that look like ordinary files.) Like an archive file, a disk image is a single file that contains all your other filesand can optionally be compressed, encrypted, or both. The difference is that you can double-click a disk image, and after supplying the passphrase (if necessary) it will mount on the Desktop as a regular volumeafter which you can read and copy files using the Finder. Sounds great, doesn't it? It can be, but keep in mind that in most cases, each incremental archive backup is stored on a separate disk image, so without a snapshot or file list provided by the backup software (see Snapshots and File Lists, just ahead), restoration can be quite involved.
When making a duplicate onto another hard disk, Finder copies are obviously mandatory. For archives, though, I strongly prefer a format that offers both compression and encryptionand in this respect, archive files are generally more elegant and convenient than disk images.
Maxtor OneTouch drives include software with a feature called DiskLock, which prevents access to the drive's contents unless you enter a password. DiskLock does not encrypt the drive's contents, thoughit merely hides them. This feature may protect your data from a casual snoop, but it won't stop a determined hacker nearly as effectively as encryption will.
Snapshots and File Lists
When it comes time to restore files from an archive, you must be able to locate the versions you're looking for quickly and easily. Some backup programs facilitate such restorations by offering snapshotslists of all the files on your computer as they existed at the time of each backup, even if they were already present in the archive and therefore not copied during that particular session. Suppose you want to restore all the files on your machine as they existed last Tuesday. Having a list of all the files as they appeared on Tuesdayand an automated way to restore themcan be extremely valuable.
On the other hand, imagine that you want to look back at every version of just one particular file as it existed over the past month. In this case, you don't want to wade through snapshotsyou simply want a list (sorted by file name or dateor better yet, searchable) of each version of the file in the archive, from which you can choose just the ones you want. Without either a snapshot or a file list, you'll need to locate each version of the file manuallyoften in a series of dated folders. This makes for a long and tedious restoration process.
Sources and Destinations
The volume from which you back up files is known as the source; the volume to which you back them up is known as the destination (or target). Be sure the software you select can accommodate the sources and destinations you wish to use.
All backup programs can copy data from your startup disk. Most can also copy data from other attached hard drives, network volumes (including AppleShare volumes, FTP servers, and iDisks mounted in the Finder). And usually you can select arbitrary folders or files anywhere on those volumes to be backed up. However, there are exceptions. Backup Simplicity, for example, supports only your startup volume.
Even if your backup software supports copying files from a mounted network server, you will generally be unable to make a bootable backup of a network volume.
In most cases, your range of destination options also includes any Finder-mountable volume. (So, theoretically you could even back up one network volume to a different network volume if you wanted to.) If you wish, you can even back up your files onto a disk image. Most programs require that you manually create the disk image yourself using Disk Utility and mount it in the Finder before you can use it as a backup destination.
A similar issue comes into play with optical media. A backup program can support recordable CDs and DVDs as a destination in either of two senses:
The first way of supporting optical media is trivially easy for software developers to implement, so that is how many backup programs work. But this approach does have some problems. First, it requires much more human interventionperforming manual steps despite the fact that the backup itself runs automatically on a schedule. Second, it eliminates the possibility of multisession recording (the ability to record additional chunks of information on a partially used disc after the initial write session) since the Finder does not include this feature. This is a problem because without multisession capability, you will use a much larger number of discsincreasing not only media cost, but inconvenience. (Note, however, that some applications, including Retrospect, use a packet-writing technique to add data to partially used optical discs. This is even more efficient than multisession support, but it means that only the application used to record the discs can read them later.) Therefore, if you need to record backups onto optical media, I strongly recommend using an application with multisession (or packet-writing) support.
A related issue is what I'm going to call media spanning. Suppose you have more data than will fit on a single CD or DVDor even that you have a single file that's too large to fit on a single disc. Some backup programs intelligently manage backups that span multiple discs, prompting you for new media when required during a backup (splitting files if necessary), and asking for the proper discs when restoring files (rejoining split files). Although the need for media spanning could affect those backing up onto hard drives as well, it's most crucial for those using optical media. Only a few backup programs offer media spanning, and even fewer include both media spanning and multisession or packet-writing support.
Selectors and Exclusions
Selective archive backups do not include every file on your hard disk. But archiving even your entire home folder may be overkill, since it includes things like cache files, which serve no useful purpose in the context of a backup, and digital media files (such as MP3s ripped from your CD collection), which, because they change infrequently, are adequately backed up by your duplicates. So instead of selecting one or more folders to archive, you may wish to explicitly include or exclude certain types of files.
Some backup programs include user-definable criteria specifying which files should be included (selectors) or excluded (exclusions) from a particular folder or volumeand a few programs offer both. Depending on the program, these criteria may include file names, sizes, Finder labels, extensions, modification dates, and any number of other factors.
In general, I find exclusions more useful than selectors, though I would not generally consider either an absolute must in a backup program. Your mileage, of course, may vary.
Ease of Restoration
No matter how easy it is to back up your hard disk, if your software makes it difficult to restore files, you're going to be unhappy with it. After all, a backup that you can't restore is worthless. Backup programs typically offer one of three main approaches to restoration:
It probably goes without saying that I prefer applications with a Restore commandthey make the restoration quicker and easier. Of course, the presence of a Restore feature does not, by itself, mean the process will be easy, but it's a hopeful sign.
Restoring a Full Archive as a Bootable Volume
If you choose to perform a full (rather than selective) archive, bear in mind that not all backup software can restore your archive from an arbitrary point to a blank disk in such a way that the resulting volume will be bootable. In order for a restored full archive to be bootable, several things must be true:
Most backup software that provides both duplication and archiving features also enables you to restore a full archive as a bootable volume, assuming that you set it up properly. Some programs, however (notably Synchronize Pro X), can restore a bootable volume only from a duplicate, not from an archive. A few applications permit full archives to be restored as bootable volumes, but lack a snapshot featuremeaning you must manually locate and copy a large number of documents to return your disk to the state you wish to recreate.
Unfortunately, most backup software does not explicitly state whether or not it can restore a full archive as a bootable volume, and of the programs that do, some are more reliable in this regard than others. This may be a good reason to consider performing selective backups instead; on the other hand, if full archives are important to you, I recommend using Retrospect.