How to fix Time Machine's "This backup is too large for the backup disk"?

This happened to me recently; I was pretty annoyed because I expected that if there wasn't enough space, Time Machine should just delete the oldest backup.

The problem turned out to be that the disk contained only one (large) backup, and didn't have room to store the current delta (delta = changes to files since last backup).

I realized this when I looked at the disk contents and saw that there was only one backup folder under Backups.backupdb.

I used Disk Inventory X to look at the contents of my hard disk, and identified a few large folders which didn't need to be backed up. I excluded these from the backup using "Time Machine" > "Options" > "Exclude these items from backups". The info from Disk Inventory X also led me to delete a bunch of large unused files.

After the two changes listed above, the backup proceeded successfully. I didn't have to launch the backup, it automatically retried. I presume that my changes brought the delta to a small enough size that it fit in the remaining free space on the backup disk.

UPDATE: This happened to me again recently, and just excluding folders from the backup wasn't enough, so I found another trick. Let's say you analyze your hard disk contents and realize there's a 20GB folder (let's call it UselessStuff) which was being backed up. You exclude it from the backup as described above, but that doesn't delete the previous backups of UselessStuff from the disk, and you still don't have enough space for the backup to complete. Since you don't need any backups of UselessStuff, you can enter Time Machine, right click on UselessStuff, and click "Delete all backups...". In my case, this freed up enough space to let the backup continue successfully.

First, I wanted to point out that it's strange that the dialog shows commas in the numbers instead of decimal points. I'm not sure what's up with that. If Chris Nava's reply didn't help you, read on.

Sometimes Time Machine gets jammed up when there is a really big change in data. I can see that you have over 12GB to back up and only 2GB available. The changes have either been building up for awhile or something large was recently changed on your drive.

Is there any data on this backup drive that is not from your Time Machine backup? Is more than one Mac using the drive as a backup drive? Time Machine from one computer can't weed out data from another's backup to make room for itself.

Are you backing up a monolithic database of some kind that changes regularly? Something like Entourage's mail database can get very large and a single new mail message tags it as completely new for Time Machine. This can cause a Time Machine Backup to fill up quickly. The Photo management application Aperture does the same thing.

Also, how large is this backup drive?

Hmm... I think that Time Machine creates a sparsebundle image which is particular size - when you changed your disk the image will not have been resized to match the increased capacity.

Apparently running the command (in a terminal window):

hdiutil resize -size 50g MyFile.sparsebundle

(where 50g is the size you want to resize it to, and MyFile.sparsebundle is the name of the image file).

will resize the sparsebundle image file. You will need to have "cd'ed" into the time machines directory for this to work.

Make sure your backup is backed up... if you know what I mean.

Of course, the above could be nothing to do with your problem - It's just a thought!

Open preferences and make sure it says "The oldest backups are deleted when your disk becomes full."

In my case, "another computer" had a backup on my backup disk.

Using Finder, look inside /Volumes/<your backup disk>/Backups.backupdb/<your computer's name>/Latest.

It's possible that you might find an entry you don't expect in addition to <your computer name>.

Or, like in my case, you look there and find nothing interesting, but fail to look in ../Latest right away which would have made you realize that your old computer (same hardware, pre-Lion clean-install) had the same name as the new one but a custom hard drive name. The backup from the old hard drive name was never re-used and never deleted.

I made sure I didn't still need anything in that backup, then deleted it. Backups have proceeded normally from there.

Good luck to all!