Time Machine failed because there is not enough room on the ext HD

Question

Time Machine has been working great for a year or so, but just today I got an error message stating that it cannot backup because the ext HD hasn't enough available space.
I was under the impression that Time Machine deleted older backup content, to make room for the new? I deleted about 30GB of stuff on my internal HD, which provided enough room for this backup, but I'm anticipating the problem to reoccur.
What's up with this? Does Time Machine automatically delete old backup content to make room for the new, or what?

Answer 1

OS Lucinity wrote:
I'm starting to think that perhaps the smaller TM partition was overwhelmed by the amount of data it had to backup. I didn't realize how quickly the 2 partitions it was responsible for filled up.
Yes. I'm still unsure how much data, total, you're trying to backup, but your TM space is clearly filled-up now.
The usual "rule of thumb" is, TM needs 2-3 times the space of whatever it's backing up. But it actually depends almost entirely on how you use your Mac. If you change lots of big files very frequently, then even 3 times may not be enough. But if you're a light user, you might get by with 1.5 times.
And no, you shouldn't have to manually delete backups under ordinary circumstances. TM will manage it's backups and space for you, +if it can.+
If you haven't seen it, TM's procedure is to do hourly backups and keep them for 24 hours, except the first of the day, which it keeps for 30 days. After 30 days, it makes one per week into a weekly backup which it keeps for as long as it has room. If it can't keep to this schedule, it will fail and you must decide what to do.
This can happen, of course, if there just isn't enough room. It can also happen if you suddenly put a large amount of other data in it's partition (it does need a fair amount of free space to work with).
It can also happen if TM needs to do a full backup instead of an incremental, which happens if you don't let it do any backups for several days (10, I think); or after a full system restore; or after certain hardware is replaced.
TM also can get confused if you change, move, or delete things from it's backups via the Finder; use the TM interface instead. There, you can delete a whole backup, or all backups of a particular file, etc.
Ok good to know, is there a "best-practices" Apple tutorial for TM?
Not really. Apple help has some, as does this article and the links in it: http://support.apple.com/kb/HT1770 but, as with most apps, detailed documentation isn't available, not even a FAQ section.
If I have to manually remove old backups, I'm more likely to find a new backup solution.
You might have to, but then you lose the ability to recover a recent previous version of a file that got deleted or changed in error, or somehow corrupted.
Your best bet is more space for TM and/or less to back up. If you have some old data that rarely if ever gets changed, you might consider copying it to DVDs, then excluding it from TM.
And I hate to add to your woes, but do you have anything off-site? As great as external drives are, they may not protect you from fire, flood, theft, direct lightning strike on your power lines, etc.