BackUp Drive : Size limitation - how to resolve?

In an effort to obtain more information please open the backup drive using the Finder. Select the mounted backup drive, then File > Open. Then, select View > As List. Make that window large enough to capture all its contents.

Then, take a screenshot of that Finder window: Take a screenshot on your Mac - Apple Support and post it in a reply to this Discussion.

• It is not necessary to capture anything other than what is displayed in that Finder window. How to capture a window or menu in Take a screenshot on your Mac describes how to capture only what needs to be shown.

To post a screenshot use the "picture" icon below:

I thought the backups are incremental backups?

Yes, they are.

If you have an aggregate amount of 12 TB of source data, you will need a backup drive of at least that much capacity to create one backup. The subsequent backup will copy only new files and existing files that have been modified, but if that means it can no longer retain copies of the files they replaced due to insufficient excess capacity, those older ones are discarded. They are discarded after the subsequent backup is finished — meaning the space they occupy will be required until then. Therefore you need more than 12 TB. The question is how much more.

A more complete explanation renders that example moot, because TM estimates how much capacity it needs prior to even beginning an incremental backup. If that capacity is deemed insufficient, TM won't even begin that incremental backup and will tell you it's out of space. In that manner, TM always has one backup from which to completely restore a system. It's very conservative in that manner.

It gets worse, because if many files become changed between incremental backups, that required space estimate becomes greater. A macOS update or upgrade for example will require many gigabytes of space.

This is the reason a lot of people recommend a TM backup drive of two or three times the source volume contents. Such a "rule of thumb" is almost guaranteed to keep most people out of trouble, but like most such "rules" it lacks mathematical justification. It just minimizes the number of callbacks and complaints.

The salient point to bear in mind is that TM guarantees an absolute minimum of one and only one complete, restorable backup.

Doesn't TM spread the backups of the items across the 2 Backup drives if it cannot fit into one?

Sadly, no, it can't do that.

If not, what's the advantage of sparing 2 HD for backups instead of one?

Redundancy. You definitely do not want to keep all your backups on one device, since any drive can fail at any time for any reason, or for no apparent reason whatsoever.

OK, so are you saying that each of my backup drive needs to have the capacity to backup all my contents (12TB) with, at least, one full backup?

Given the above longwinded explanation, yes, and in fact you are likely to need considerably more than 12 TB. 16 TB might be ok though, for the reasons I explained. The aforementioned "rule of thumb" would have you in need of a 36 TB backup drive. That example demonstrates its fallibility.

ArtyYeo wrote:

1. I am currently using two HDs for TimeMachine (TM) Backup:

12TB

2. 16 TB

Lately, I've been getting an error message from TM saying that there is not enough space to do a backup on the smaller 12 TB HD.

My understanding is that TM is supposed to overwrite old backups when there is not enough space, correct? Why not enough space?

This begs a question: eventually, the 16 TB HD will also hit this ceiling. Then, what am I supposed to do? there are hardly any 20TB HDs out there.

Without knowing the or provenance of the drive or its history, it is hard to speculate—

you can however reformat the drive and start the backup over.

GUID/apfs

Erase and reformat a storage device in Disk Utility on Mac

Erase and reformat a storage device in Disk Utility on Mac - Apple Support

3-2-1 Backup Strategy: three copies of your data, two different methods, and one offsite.

One possible explanation is that the backup drive may contain backups from what it considers a different Mac. For example, if you were to change the name of your Mac (or if its name spontaneously changed for reasons unknown) TM will begin a new set of backups and leave the ones from "the other Mac" intact and unalterable.

The solution is as leroydouglas suggested.

Are those external drives also being backed up? If so you found the problem.

TM's default is to back up all mounted drives (except for the TM backup drive itself). If you do not want TM to back them up, add them to its exclusions list: Exclude files from a Time Machine backup on Mac - Apple Support.

Even though the title refers to "files" it should really say "items" as explained in that document: Step 3.

I'd keep an eye on it in an effort to determine how much it's backing up each time.

How large is the source volume's contents? Not its capacity, which is irrelevant.

I assume no junk products are affecting that Mac: no "anti-virus" / "cleaning" or similarly categorized junk. That just goes without saying.