Q I have a 3 TB external RAID system which stores Time Machine backups for my 2 TB iMac. Free space on the backup drive has generally been around 700 or 800 GB over the last few months, but just recently it has started to fall extremely rapidly, reaching only 500 GB now. Other than the recent stack of app and OS X updates, the only thing which has changed is that I have installed Parallels in order to run a Windows app a couple of times each day. Why has Time Machine suddenly started to consume free space on the backup drive?
A For Time Machine to consume large amounts of free disk space on the backup drive, it must have been making repeatedly large backups. Using Console, browse back at its most recent backups listed there and you should get a good idea of the space required by each, as recorded in its log entries.
The most likely issue is that, now you are using Parallels on a regular basis, Time Machine is making daily backups of your Windows virtual drive, normally located in the ~/Documents/Parallels folder. Open that folder and you will see a file named Windows 8.1.pvm, or Windows 10.pvm, or similar, which is at least 18 GB in size.
Each time that you run Windows in Parallels, that file will be changed, and will therefore be flagged to be backed up the next time that Time Machine runs. If that happens twice a day, in just ten days you will have 360 GB of backups of that file alone: sufficient to eat an obvious and growing hole in free disk space.
If you really need a backup of that Windows virtual drive, you may be better performing that manually, when you need, and removing it from the Time Machine backup.
To do the latter, open the Time Machine pane, click on the Options… button, then add the ~/Documents/Parallels folder to the list of items excluded from backups, by clicking on the + tool. Save that, then open the Time Machine app, navigate to the ~/Documents/Parallels folder, and select it. Then bring up the contextual menu, and use its command to remove all backups of that folder. You should find the free space on your backup drive revert to its former size.
Comments This is a useful technique for more generally controlling the growth of Time Machine backups, and ensuring that sufficient free disk space remains on your backup drive: identify a very large file or folder which quite often gets backed up, but which does not need to be part of your routine backup, then exclude it from Time Machine backups, and remove it from your existing backups.