Frustrations of Time Machine recovering storage by deleting backups

As I understand it, you have one external backup drive used for Time Machine backups in addition to other, non-Time Machine storage. That is not a good idea. First, TM expects to use the entire capacity of the volume dedicated to it. That means you will reach a point at which you will not be able to add any more, non-TM files to that disk.

As far as those other, non-TM related files on that disk, TM will respect their presence and will never delete them, but those files are not backed up. Losing that drive for any number of reasons is a possibility, so consider that a factor in your backup strategy.

Summary: get a separate drive for non-TM storage. TM will back it up too, if it is mounted when TM is backing up and you do not explicitly exclude it from TM.

Is it safe to use the Tmutil delete path.?

I don't know what that means. As for the other questions, deleting "all backups of <...>" will take a very long time. If that process is interrupted it increases the likelihood of corrupting the TM backup, rendering it useless.

I can think of few if any reasons for "deleting all backups of..." a file. One might for example wish to ensure the deletion of some sensitive documents and their backups for whatever reason, but as long as the TM backup is encrypted along with its source volume, there should be no concern they could be retrieved. You would need its password, without which the backup files cannot be read. Conceivably, that backup could be copied to another volume, but it would remain encrypted.

Is there a way to limit the size of the Backups.backupdb on a multi-use external hard drive?

No.

At least if the space is defined, the next backup by Time Machine should delete the oldest backup.

That's a close description, but it doesn't work quite that way. TM deletes old and "expired" backups according to an algorithm that is not publicly documented or even explained very well. It does that when it needs the space they occupy. It is not always the oldest backup that gets deleted. That will occur eventually, but it is impossible to predict when.

I suspect you may be intending to use Time Machine as an archival system. That's a concern. It is not that, and never has been. TM guarantees an absolute minimum of one and only one complete, restorable system backup. As such, if you have not encountered a need to recover a file that you deleted from your Mac more than a few months ago (for example), it's likely you will never encounter that need.

TMUTIL ‘route’ that was suggested by several people but have not tried it yet.

I frequently come across absolutely horrible advice posted to this site from longtime Mac users who ought to know better — even from those who readily admit to not using Time Machine — probably due to very frequently parroted misinformation. Even if the information was correct at the time, Apple is constantly implementing changes to Time Machine. I have never known them to describe or even acknowledge those changes.

Don’t know whether the old Catalina TM backups are viable for the new Monterey MBP.

They should be, but I would lack confidence in that fact unless I were to try it myself (and I have). Performing a test restoration is a very good idea. Trust but verify. Have a contingency plan of course.

If you want to preserve older backups, my recommendation is to remove that device from Time Machine (use its "Add or Remove Backup Disk..." option to remove it) until you are confident it will no longer be required. Whether that takes weeks, months, or years makes no difference. Then, erase that drive — the entire device, not just a backup folder or sparsebundle disk image — and start a new set of backups on it. That's what I do and what I recommend.

If you should change your mind prior to erasing it, simply adding that backup disk again will cause TM to pick up where it left off as if nothing had happened except the passage of time, with the probability of many changed files that need to be backed up.

Whether such a practice is Apple's recommendation or requirement doesn't matter to me. They're my backups, and no one else will care about them as much as I do. Besides, I happen to know that Apple made claims about TM in the distant past that were simply not true. They were never true. Apple doesn't say "sorry about that, we were wrong", they just quietly remove those references and documents. That historical perspective has led me to avoid "reading into" anything Apple does not explicitly state.

Use the following as you wish. TM will cycle its backups among as many drives you wish to provide, so here's an example:

At the time those two drives were placed into service, they were dedicated exclusively to TM. Notice how both of them are "read only". I did nothing to make that occur; that's just the way TM configured them. It would be a simple matter to authenticate and change them to read / write, but why would I do that?

Out of interest, what size TM dedicated disk do you / Apple suggest???

"How big is big enough" is a simple question that eludes an accurate answer that works for everyone in every case, but it's simple enough to arrive at an approximation: Time Machine requires enough space to retain an absolute minimum of one complete, restorable system backup plus sufficient space to finish a subsequent backup consisting of the aggregate amount of information changed from its previous backup, plus an additional amount of "overhead" that is not easily calculated for a number of reasons.

If you were to consume 100% of a source volume's capacity (which isn't really possible), an external TM backup drive of twice that capacity ought to be sufficient. You can certainly get away with less, but more would be nice. It comes back to the question of how many months or years of backups you really need, and everyone's needs are different.

Do you have a question about Time Machine?

I've been using Time Machine since its inception about a decade and a half ago, through all the intervening macOS updates and upgrades. It works perfectly if you let it, but you can definitely run into trouble if you try to micromanage it.

If you are having trouble I can help but if you simply want to lodge a complaint then you should tell Apple.

👍

My pleasure.