Time Machine not working: not enough space on the backup disk?

[EEzycade]

I've had a Sandisk 500gb drive that I've used as a time machine backup drive for about 4 years. Recently, I upgraded to a 14" MBP w/ M1 Pro. Since using the new computer, Time Machine worked for some time then stopped working for some reason. It was also interesting that for a long time, I had to enter a password to access the Sandisk drive. When it stopped asking for that, is when it stopped working for time machine entirely. So I erased the Sadisk drive in Disk Utility. Then tried to back up to it. It was going well until it failed and said there wasn't enough space on the backup drive. That's really strange because I've only used about 400gb on my 500 gb SSD in the MacBook Pro. And the sandisk is also 500gb. Why is it saying there's not enough space?

It's also weird because in System Preferences > Time Machine it says Oldest Backup: None and Latest Backup: None. But in Disk Utility, it shows the Sandisk as having used 460 out of 499 gb. How is that true if there's zero backups?

 

[NoBoMac]

Not sure what exactly is going on, but a 500GB external drive is not sufficient for a Mac with a 500GB drive. Apple has recommended in the past that a Time Machine drive should be at least 2.5 times bigger. So would want a 2TB drive at minimum.

Sounds like Time Machine was backing up and stopped when it realized it could not complete the backup. Space is being used by a partial/incomplete backup that is unusable, hence no backups.

And a 500GB drive does not actually have 500GB available as the formatting process uses up some space to hold volume information/tables. Each file that gets written has overhead, the snapshot that is the backup has overhead, etc.

So, seems like you are using a fair amount more than 400GB on the Mac and the external is too small to be a viable TM drive (the culling process will be constantly flushing "old" files due to lack of space, so not getting multiple backups of files). That drive could be used for Carbon Copy Cloner and ilk clone destination, but pretty much worthless for TM.

 

[Brian33]

I can't say for sure what's wrong, but I think that there was (is) data still on the drive even after you "erased" it. I believe you may have erased only one APFS volume, but not the entire drive (hardware device) itself. This can easily happen because by default Disk Utility shows only volumes, and not the devices (or containers!). So, there could have been more than one volume in the APFS container -- perhaps one that's hidden from the desktop so it would be easy to overlook.

Or, perhaps the Sandisk drive has more than one partition, either accidentally or purposely? (If it's purposeful, you'll need to store any data you want to keep in another location!)

In any case, the remedy should be the same -- repartitioning and reformatting the drive, being sure to select the top-level device before clicking Erase:

1. First, open Disk Utility, go to the View menu and make sure Show All Devices is checked! (By default it's not.)
2. Now in the left pane, select the device that corresponds to you Sandisk external drive. It should be under "External" and it will probably have "sandisk" in its name. There will be a few lines "indented" under that Sandisk device, representing the APFS container and volume. (In any case, DON'T pick the APPLE SSD line -- that's your internal storage device!)
3. Having selected the proper line, now click Erase.
4. In the window, make sure the Scheme is GUID Partition Map. I don't think it matters if you format it APFS or MacOS extended, because Time Machine will re-format it at APFS anyway before using it.

This proceedure will result in the drive having a single partition, with a single volume, if you selected macOS Extended, or a single partition, single container, single volume if you selected APFS format.

Not sure what exactly is going on, but a 500GB external drive is not sufficient for a Mac with a 500GB drive. Apple has recommended in the past that a Time Machine drive should be at least 2.5 times bigger. So would want a 2TB drive at minimum.

I haven't tried it, but I think TM could successfully back up to an external drive that's equal to the boot storage. At the least, the first backup should succeed. If the boot drive really only has 400 GB in use, there should be enough room for a few older snapshots. Also, by default, quite a lot of data is excluded by TM, like cache files, spotlight indexes, trashes, and the System Volume itself (about 16 GB). So I think it could work.

Sure, it's not ideal, as not much history would be kept, but I think TM should be able to handle this situation. As you say, though, perhaps CCC would be a better choice (but not as easy to set up ).

 

[Fishrrman]

NoBo said:
"Not sure what exactly is going on, but a 500GB external drive is not sufficient for a Mac with a 500GB drive. "

Ahem.
It can be if you're using CarbonCopyCloner or SuperDuper (instead of time machine).

I backed up my internal drive for years with a backup drive that was SMALLER than the internal drive -- again, using CCC.

To the OP:
Either
a. get a bigger drive for your backups
or
b. erase the existing backup drive and start over.

Best solution:
STOP USING tm, and START USING either CCC or SD.

My opinion only
Others will disagree
Some will disagree vehemently

 

[EEzycade]

Thanks all for the replies, they were very helpful!What I ended up doing was buying a 1tb Samsung T7 and using that for Time Machine, worked like a charm! Using the old ssd as a dumping ground for recordings of Nintendo Switch gameplay.