Time machine not recognizing free space even though it "knows" about it

Question

Trying to back up to an external hard drive I'm getting this error from Time Machine when it tries to back up:
"This backup is too large for the backup volume. The backup requires 52.5 GB but only 495.8 GB are available..."
Clearly it KNOWS the space is there, but its math is wrong since it seem to think that 52.5 GB is greater than 495.8 GB.
Suggestions?

Answer 1

Sometimes, this error is legitimate. The user may not realize that they need to exclude additional hard disks that TM may be seeing. This is done in the “Options…” window of the Time Machine Preferences.
Nevertheless, many times this error just doesn’t make sense, stating that not enough free space exists for a backup that is clearly smaller. (“The backup requires 45.9 GB but only 182.6 GB are available”)
The answer and fix may have been discovered by a poster in the macosxhints forums: (http://forums.macosxhints.com/archive/index.php/t-84284.html)
Essentially, while Time Machine DOES delete old backups from the sparsebundle, it DOES NOT free up the space occupied by those old backups. Eventually, "all previous backups were deleted and no space was gained. . . . What appears to be going on here is that the sparsebundle isn't self-shrinking - unused bands aren't being automagically discarded."
The solution is to compact the TM backup sparsebundle in Terminal using:
hdiutil compact yoursparsebundlefilename.
He concludes, "I gained 30+gb of space on the backup volume by compacting that sparsebundle. And now TM runs just fine, since it has plenty of space."
Obviously, this is a bug in TM. It does not appear to effect everyone, and only seems to appear under certain curcumstances.
Let us know if this works for you.