Time Machine is Determined to Delete Old Backups to Make Space it Doesn't Need Okay, so my machine suffered a power loss this morning, and when Time Machine went to back-up it seems to have decided it can't trust the usual event log that it uses:

02/10/2014 14:56:43.351 com.apple.backupd[63107]: Event store UUIDs don't match for volume: Macintosh HD


It then proceeds to do a deep-scan, which is understandable I suppose.

02/10/2014 14:56:43.428 com.apple.backupd[63107]: Deep event scan at path:/ reason:must scan subdirs|new event db|
02/10/2014 15:04:58.809 com.apple.backupd[63107]: Finished scan
02/10/2014 15:10:53.693 com.apple.backupd[63107]: Found 402101 files (1.8 TB) needing backup
02/10/2014 15:10:53.747 com.apple.backupd[63107]: 2.16 TB required (including padding), 1.04 TB available


However, the required space estimate is far too high; my back-up volume's total size is only 3tb, and the total data on Macintosh HD is around 2tb, this means that if I let Time Machine continue to run it's going to delete all of my backup history and then fail with a warning that it can't backup because there isn't enough space.

This is clearly wrong however, as the back-ups on the disk are valid, and should be matching most of the files found on Macintosh HD.

Is there a way I can get Time Machine to recognise this and adjust its estimate accordingly? If not then I'm going to have to start fresh with a new back-up, but that will take days to get up to date again!

[edit]
I've just tried using tmutil associatedisk -a and tmutil inheritbackup to no avail, after an even longer deep scan it just starts trying to discard old backups again.
It turns out my particular problem wasn't covered by other, similar cases, so I'm going to post my own answer describing the actual issue I had, and the solution I found to it.

In my case it appears that somehow my last "complete" Time Machine backup was in fact corrupted somehow; when I looked inside it I found that the contents of my user folder were missing, so Time Machine was in fact correct in thinking that it needed to backup everything from my user folder again.

To solve this I needed to delete the last "complete" backup on my Time Machine drive. To do this I opened my Time Machine volume in the Finder, and navigated inside the Backups.backupdb folder, my computer name and then found the folder with the most recent time and date. After taking a quick look at some of the largest folders I saw that my user folder (the biggest on my system) was empty.

To fix this problem I then ran the following command in Terminal:

sudo tmutil delete "/Volumes/Time Machine Backup/Backups.backupdb/MyComputer/2014-10-01-175432"


Of course you will need to replace the part in quotes with the correct path to the corrupted backup. You can also do this easily by instead typing sudo tmutil delete (with trailing space at the end) and then dragging the folder onto the Terminal window to have it add the path for you (it may include backslashes, this is normal). You'll need to run this command using an administrator account and enter its password, since only an administrator can alter Time Machine's backups.

For anyone experiencing a similar issue to what I was seeing, I would now highly recommend giving your latest backup a quick check to make sure that you aren't experiencing the same problem I was; although the chances of this corruption occurring seems extremely low (Time Machine would have to be forcibly interrupted or crash for it to occur, though having your Time Machine volume disconnect may do it), it doesn't take long to give your latest backup a quick check, but it can save you a lot of time before you go ahead and try other possible solutions.

If you aren't experiencing the same problem I did then I recommend trying the following links:


Time machine wants to backup too much
TM is doing a full backup for no good reason
My backups seem too large