I've long recommended SuperDuper! for mirroring Macs to an external drive, but I've recently been having quite a bit of trouble with the application and may have to rethink my recommendation going forward.
Specifically, the problem I'm having is that the external drive is completely filling up, seemingly at random and despite the fact that the drive I'm mirroring is only about half full. This has happened three times in the last few months. I'm using the "smart update" feature which, like any good mirroring program, updates the external drive only when something changes on the Mac. In other words, it maintains the mirror incrementally instead of doing a full wipe + copy each time. The advantages of this method are obvious: much faster updates (usually ~30 minutes for me), less wear and tear on the drives, and less chance you'll lose all of your data — except when it doesn't work.
After having dealt with this issue a few times, I did a little research and quickly found out I wasn't alone. Unfortunately, there is no fix and the developer's explanation is a bit weak. From the user guide:
If the destination volume is actually full, and the source volume should definitely fit on the destination, this could be because Smart Update needs more “working space”. This is because Smart Update works its way through a volume in a single pass, and copies or deletes files and folders as it encounters them. If you rename a large folder or file, or add one folder and delete another, Smart Update may encounter the new file or folder before it deletes the old. In that case, it temporarily needs enough disk space to hold both the old and new versions.
While this situation may apply to some people, it certainly doesn't explain why I'm having trouble, because (1) I'm not using my computer at all when SuperDuper! runs at 4AM every morning1 and (2) there is more than enough "working space" (as I said, both the source and destination drives are less than half full and no single file is anywhere near as large as half the drives' 250GB capacities). I just don't understand how a backup program can let 125GB of meaningless data trickle onto a destination drive over the course of three months.
To be as fair as possible, I'll reiterate that this has only happened to me three times in the last few years, but I'll still tell you that that's three times too many; backing up my data is something I take very seriously — it doesn't get much more mission-critical for me. Am I just going to have to revert back to rsync?
Sure, there may be some background processes doing some maintenance work or the occasional downloading of a large file that's still chugging along at 4AM, but none of that activity should ever threaten to fill up the half-full destination drive. ↩