Currently Being ModeratedMar 25, 2013 1:56 PM (in response to iPod Touch - Wipe)
A Time Machine backup drive needs to be at least two or three times the capacity of the drive backed up. For example, if your computer's hard drive is 250 GBs, then a Time Machine backup drive needs to be between 500 and 750 GBs. The larger the backup drive the less frequently you will fill up the drive and see:
About TM "Backup Drive is Full"
Alert TM only deletes older files if they have been deleted from the source and when TM needs space on the backup drive for a new incremental backup. Time Machine "thins" it's backups; hourly backups over 24 hours old, except the first of the day; those "daily" backups over 30 days old, except the first of the week. The weeklies are kept as long as there's room.
So, how long a backup file remains depends on how long it was on your Mac before being deleted, assuming you do at least one backup per day. If it was there for at least 24 hours, it will be kept for at least a month. If it was there for at least a week, it will be kept as long as there's room.
Note, that on a Time Capsule the sparsebundle grows in size as needed, but doesn't shrink. Thus, from the user's view of the TC it appears that no space has been freed, although there may be space in the sparsebundle.
Once TM has found it cannot free up enough space for a new backup it reports the disk is full. You can either erase the backup drive and start your backups anew or replace the drive with a larger drive.
Currently Being ModeratedMar 25, 2013 2:08 PM (in response to iPod Touch - Wipe)
One thing that's helpful to remember is that TM keeps a copy of every different version of a file, so large files which are frequently modified can take up a lot of space in a TM backup.
My strategy is to move files which are no longer needed (like finished projects), files needed only for reference (manuals), software installers, and the like, to a separate Archive drive which is automatically cloned as a backup, and use Time Machine only as a backup to restore part or all of my startup drive.
In this configuration I have a 500GB Time Capsule for Time Machine, which keeps more than a year of backups of the 250GB or so of data on my startup drive. That is the low end of the recommended size for a TM drive (2x amount to be backed up), but in my view there is no sense using up TM space for "archive" type files which are for reference only.