Time Machine manages its space for you, automatically.   When its drive/partition gets near full, it will begin deleting your oldest backups to make room for new ones. See item #C4  for more info and your options. 

In addition, regardless of space, Time Machine also routinely "thins" your backups. It keeps one per week for as long as there's room; one per day (the first) for a month; all others for 24 hours.

Usually when many old weekly backups are deleted unexpectedly, it's because Time Machine has done a new full or large backup, which of course requires a lot of space. See  section #D3  or #D4  for common reasons.

If in doubt, get the Time Machine Buddy widget messages (see section #A1).  They'll show how much it was trying to back up. See  item #A6  for explanation of those messages.

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Copyright © 2011 James Pond.  All rights reserved.