My 64-bit Vista Ultimate box started running out of disk space on the backup hard drive as with others before me, so I had to spend some quality time investigating the details. Just in case you are new to backups: it is a complicated subject and Microsoft tried to simplify it for consumers by offering a quick automatic backup option. The reality is that by flipping on the backup switch and walking away you will most likely fall into the leaky abstractions trap. [In his great article Joel does not even mention that on occasion a CRC and the TCP checksum may disagree. So it is even worse than it appears.]
Notice that Microsoft may expose two radically different backup technologies (Backup Files and Backup Computer) in the more advanced Windows flavors; the average consumer may only use Backup Files. It was not a good sign to discover after some Googling that the matter drove some to utter profanity.
What happens if Windows detects large files to back up, potentially on a daily basis? What if Google Earth, to improve user experience and minimize network traffic, saves a large disk cache (in the order of 1 GB)? What if Google Picasa saves a large disk cache with the previews of your pictures? What if Microsoft Windows itself constantly updates a giant database that is caching thumbnails (ThumbCacheToDelete)? It appears that nobody tried to address these problems up front, so it is now our problem.
I downloaded TreeSize Free to understand who was using disk space the most. [You can also use WinDirStat if you prefer.] Sure enough, these were the largest offenders. I even found a stale 1-GB copy of the Google Earth cache in a previous location — apparently at some point they decided to move the default location to the LocalLow area.
As explained in this Guide to Windows Vista Backup Technologies, with Backup Computer (a.k.a. Complete PC Backup)
only new and changed data is written; however Backup Files will not do this: in other words, if a small update is applied to a huge file, Backup Computer will record only the differences but Backup Files will record the entire file all over again. If this happens and you let backups run unattended, large files that you may not necessarily care about will eventually fill the backup disk. Because nowadays hard drives are quite large, it may actually take months before backups start to fail. And when the emergency occurs, it appears that you will not be able to prune the backup by surgical deletion of the large files only. That’s right: you can safely delete entire backup sets, and by default only a single backup set exists:
File backups aren’t deleted automatically. However, you can delete file backups manually if you’re low on space. A word of advice: you should always delete an entire backup set as opposed to deleting individual incremental backups.
At this point it will not be hard to believe that Windows 7 vastly improves on the backup functionality.
Post Scriptum: Backup Computer (with its very space-efficient implementation) cannot be used as a complete replacement for Backup Files because it appears to be much (up to two orders of magnitude) slower under similar circumstances.