Open Radar

 
Summary:
I back up my laptop to an external hard disk whenever Time Machine notifies me that too much time has passed between backups.  However, I can no longer perform a back up; each time I try I am told that there is insufficient space.  In settings, it says that "The oldest backups are deleted when your disk becomes full".  Those two statements are contradictory; my machine is lying to me.

Steps to Reproduce:
1. Set up Time Machine to back up to an external hard disk.
2. Plug the disk in whenever prompted, and wait for the backups to finish.
3. After a few months, backups start to fail.

Expected Results:
The oldest backups should be deleted when the disk becomes full.  That's what it says it's doing, so that's what it should do.

Actual Results:
After some time "preparing the backup", the following error is displayed:

Time Machine couldn’t complete the backup to “Time Machine”.

The backup disk needs 42.79 GB for the backup but only 11.74 GB are available. Select a larger backup disk or make the backup smaller by excluding files.

Version:
OS X Yosemite 10.10.2 (14C109)

Configuration:
Macbook Air (13-inch, Mid 2012) with 256mb SSD
320gb WD external disc

Notes:
It's not so much the failure to back up my hard disk that disappoints me - it's the lying that really makes me angry.