I’ve been using Time Machine (TM) for about a year now and the drive I have to store the data is just over half full. I’m wondering how TM will handle a full (filling up) disk and if I need to do anything in the near future. I recall reading that you shouldn’t let the data disk that TM uses to go past 50% full. Not sure if that’s true, but that’s why I’m asking about it. What will happen as the disk gets full? Will it, at some point, just start erasing the oldest backup? Is there any magic to keeping the disk less than full? Should I start deleting from the oldest backups forward to keep the disk under a certain percentage full?
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How does Time Machine handle a full disk?
- Thread starter ger19
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Time Machine will erase the oldest backup first. Because they are full and incremental backups it’s best to let Time Machine manage the process automatically.
I think the 50% "rule" you read is that your data shouldn't exceed 50% of the back up disk's capacity, which is really just guidance. I.e., if you have 1TB of data (a full HD/SSD) you should have at least a 2TB backup disk. I'm not sure the "rule" is right, but you definitely want to make sure your backup disk is a good amount larger than your working disk. If the data to be backed up exceeds the capacity of the backup drive, the backup will fail and nothing is backed up.
Don't attempt to manually delete your older backups. Time Machine works by creating incremental backups after your first successful backup; all subsequent backups only register things that have changed since the last backup, so all newer backups reference the previously backed-up information. Deleting an older backup is like removing a card from the base of a house of cards. The automatic deletion used by macOS compresses the incremental backups into a new base backup, leaving any subsequent backups stable. I have seen people trying to manually delete 'old' backups essentially destroy all of their usable data and have to create a new backup from scratch.
It will delete some of the oldest backups, but at some point (I don't know when/why) it will just tell you that the "disk is full" and TM won't run on it anymore. That just happened to me with an external 2TB HDD. I lost the oldest 4-5 months' data before the disk was "full" (though with still ~80GB free), and there was no notification that old data were being deleted.
Fortunately it's not a problem because I was just learning macOS and mostly experimenting at the beginning; but if you do care, I suggest you have a new TM drive in hand when your present one gets around 75% full, and start monitoring it more closely when it does.
Originally I had the impression that TM would keep deleting old backups kind of like a dash cam overwrites the videos on its SD card over and over -- but that's not how it works; at least not for me. I have an 8TB HDD now, so hopefully I won't have to worry about it again for 3-4 years.
I agree with everyone else's mentions that the oldest backups are automatically removed and reconciled, and since the utility carefully manages and keeps track of all that, you should not tamper with the data it stores.
In my experience, a backup disk with a capacity equal to the total capacity of your actual disk(s) is sufficient for a single, current backup – but the further back in time you want to be able to restore versions of files, the larger the backup disk you should use.
I can say I've never experienced this using Time Machine since its origin in 2007, so I'm gathering something unintended happened in that case. (In my current case, for example, I've been using Time Machine with this Mac mini for almost three years, and Time Machine reports retaining backups from now back through March of this year.)
In my experience, a backup disk with a capacity equal to the total capacity of your actual disk(s) is sufficient for a single, current backup – but the further back in time you want to be able to restore versions of files, the larger the backup disk you should use.
I can say I've never experienced this using Time Machine since its origin in 2007, so I'm gathering something unintended happened in that case. (In my current case, for example, I've been using Time Machine with this Mac mini for almost three years, and Time Machine reports retaining backups from now back through March of this year.)
Interesting. So you think it should have kept replacing old backups with new, like I was thinking it would. I do manual backups twice a week; I wonder if that might have something to do with it. My solution now, for a 1TB internal drive with ~400GB of data on it, is to use an 8TB TM drive and not worry until it's 75%-80% full.
I don’t know why — seriously — but HDDs vs SSDs perform differently with Time Machine. Speed aside, I have yet to see this “disk full” alert/error after moving to SSDs.
For me, I didn’t have a concern starting over, that is, the extended backups were not a problem to lose. So, when the “disk full” problem occurred, I simply reformatted did a 'fresh' Time Machine backup from that point on.
I had a 1TB 2012 iMac, and used a 4TB external as my TimeMachine drive, so plenty of space for an initial backup and lots of incremental updates. After a few years I got that "the disk is full" notification. I'd noticed that the backup activity icon hadn't spun in a while. When clicking on it, it told me the disk was full and to clear out some space. There was no other notification, so I could easily see people missing it.
I would hope that if you tell TM to kill the older backups, after it forces you to do so, it will do it in a way that will combine the older incremental backups such that the backups are still usable... If not, it's kind of a farce, making you think you have a backup when you don't. I have never tried restoring from the backup, so I don't know whether it remains viable or not. I'd be pretty pissed off to find the unexplained forcing me to remove old backups manually killed the viability of my backups years later.
Vaguely related, the iMac would occasionally not re-mount the TM drive after waking (only like a few times a year), and clicking the TM activity icon when noticing no activity in a while it would say the last backup was some number of day or weeks ago, and I'd have to unplug and replug in the drive to get it to start backing up again. I see disk disconnect issues continue to be a topic in the forums... it really seems like something Apple needs to address.
I would hope that if you tell TM to kill the older backups, after it forces you to do so, it will do it in a way that will combine the older incremental backups such that the backups are still usable... If not, it's kind of a farce, making you think you have a backup when you don't. I have never tried restoring from the backup, so I don't know whether it remains viable or not. I'd be pretty pissed off to find the unexplained forcing me to remove old backups manually killed the viability of my backups years later.
Vaguely related, the iMac would occasionally not re-mount the TM drive after waking (only like a few times a year), and clicking the TM activity icon when noticing no activity in a while it would say the last backup was some number of day or weeks ago, and I'd have to unplug and replug in the drive to get it to start backing up again. I see disk disconnect issues continue to be a topic in the forums... it really seems like something Apple needs to address.