[jretzner]

I am running OS 1010.5 on my iMac 2.8GHz with Intel Core 2 Duo. A pop-up window appears telling me "You backup has failed." The Time Machine icon in System Preferences tells me that I have 10.4GB of 500GB available. I'm under the impression that when one's external HD become full using Time Machine that the oldest files are removed to make room for new backups. Might I have turned an option to do this off? How might I remedy this?

Thanks!

[reader50]

How big is your internal drive + any other drives you are backing up to Time Machine? How full are they?

If the amount to be backed up exceeds 500 GB, you'll need a bigger backup drive. Or exclude some things from being backed up.

[jretzner]

Okay, that I can do. But, what about Time Machine automatically erasing the oldest files to make room for newer files being backed up (or is that a figment of my imagination)?

[reader50]

Time Machine will erase older backups to make room for newer. But that only works if there is enough space for at least one backup.

You didn't answer my question - how big is your main drive, and how full is it?

[jretzner]

My internal HD has 872.61 GB free of 999.35 GB capacity.

My external HD has 104GB free of 499.97GB capacity.

An interesting thing happened this morning. I restarted my iMac because a program was stalling, or not launching. As soon as the restart was complete the program launched with no difficulties. I checked my Time Machine HD and it showed the same numbers. Suddenly it began a count down to begin a backup and completed it! Might restarting have cleaned up some glitch that was blocking backups ?

[reader50]

It's possible some process (Spotlight perhaps) was keeping too many files busy when TM tried the first time. I assumed the backup drive wasn't big enough, but we never checked the exact error message in Console. Reboots fix many peculiar issues.

So you have ~126 GB used on the main drive, backing up to a 500 GB external. That should work fine, assuming the external is only used for Time Machine. The rule of thumb is a Time Machine backup drive should be at least 50% bigger than what you're backing up. So your 500 GB should be enough until/unless your main drive reaches 333 GB used.

Is your external reserved for TM, or do you have files on it? Anything stored there (including trashed files) reserves space.

[jretzner]

I have two external hard drives going for backup. One is only for Time Machine, as you recommend. The other is used for SuperDuper. I will begin emptying my trash more often

[reader50]

In TM options, you should confirm your SuperDuper external is excluded from TM backups. Externals usually default to excluded. But if it isn't, your SD backups will be duplicated by Time Machine on a regular basis. This will produce lots of disk activity, and will eventually limit how far back your TM backups go.

Double backups.

[P]

This sounds like you ran out of space on the backup drive. It is a known issue with Time Machine, actually. What happens is that Time Machine first calculates how much space it needs, then does its "thinning" - that is, removing old backups to make room for newer ones - and finally starts to backup. The issue is that if the amount to back up changes while the backup or thinning is running, the backup process might run out space while backing up. At that point, you get that message. Time Machine then tries to fix the issue by more aggressively thinning before the next run, but unfortunately, it can get stuck in a loop, never quite removing enough. Happened to me once. Happy that it only took a reboot to fix for you.