How can I free up space by deleting old time machine backups?

Consequently, I'm thinking about picking up a second external drive and working out some kind of week-on/week-off swap of TM target drives.

Exactly. But there is no need to concern yourself with that sort of tedium. You can leave all of them connected simultaneously, or none, or one... it really makes no difference. TM keeps track of what's backed up and where. Local Snapshots remain active even when no TM backup devices are available.

Any macOS app developer can implement autosave and versioning:

"When Auto Save is enabled, a version of the document is automatically saved at least once per hour, and each time the document is opened, duplicated, locked, renamed, reverted, or manually saved. At any time, the user can browse through these saved versions and revert back to one."

Apple products like Numbers have included it for years. No one should be burdened with the tedium of explicitly "saving" their work lest they lose it. iOS implemented autosave in its inception over a decade ago. If Microsoft hasn't implemented it by now, I have no explanation.

Storage is so cheap these days, that kind of redundancy is worth the $$$.

👍

Hi Bob,

Welcome to Apple Support Communities. I understand you'd like to know how to safely free up some space on the external drive you use for Time Machine backups. I know it's important to ensure you have enough space while keeping your important backup files in tact; I'd like to help. First, I'd recommend reviewing the information in the following resource regarding a Time Machine and a full backup disk: If the Time Machine backup disk for your Mac is full

If you'd still like to remove some files to create space on your disk, the following resource has steps for how to safely do so: Delete a file from your Time Machine backup disk

Have a great weekend!

I'm not satisfied with my replies. I think that discomfort arose with the original question "how can I free up space by deleting old time machine backups" so allow me to augment my earlier answer 🙂

The question implies a perceived need to free up space, which is not required for TM to work. Apple's Support document is totally correct, but it just describes methods for deleting TM backup files. That's all. Manually intervening with TM to create space is never required, so why would anyone want to do it? Why did Apple publish that document?

The only answer I can think of is a need to eliminate possibly sensitive information contained in files already deleted from the source volume, and the user wants to make sure all copies of it are gone including its potentially many backups. It's fine for that purpose.

On the other hand deleting TM files for the sole purpose of creating space is never required.

Unless Time Machine reports it is unable to back up your Mac, you don't need to do a thing.

It is entirely normal and expected for a TM backup disk to reach capacity, after which it will begin to delete older, "expired" backups. It will continue to back up as it always has.

TM will only tell you it's beginning to delete older backups if you selected that option in its Preferences > Options. Those backups could be years old.

On the other hand if Time Machine tells you it cannot back up then something is wrong. If it hasn't, do nothing.

Thanks; that's a possible indication that something is wrong, but the fact subsequent backups seem to be occurring uneventfully then I'd tend not to be concerned. Perhaps the external drive "goes to sleep" and isn't waking fast enough for TM and it gives up prematurely, or perhaps its power became momentarily interrupted in the middle of the night, but those are merely guesses.

Your first priority ought to obtain an additional backup device and start an additional, redundant TM backup, assuming you don't have one already. It does not need to be connected simultaneously, nor does it have to be as large as your existing one, but it can be. TM will back up to as many devices as it has available.

The reason for that recommendation is to address the possibility the 6TB external drive is beginning to fail. One and only one backup doesn't comprise a robust backup strategy for the simple reason drives can fail at any time, sometimes without warning.

The fact it's 90% full does not bear on the problem at all. It's normal and not a concern. Every TM backup volume will eventually approach (but not quite reach) 100% capacity, after which will begin to delete old backups. At the other extreme, bear in mind TM guarantees an absolute minimum of one and only one backup that it will never delete. A 6 TB backup drive is likely to contain many more. Since most Macs don't have anywhere near that amount of internal storage I'd be surprised if TM were to inform you it could not back up due to insufficient space. That's a different dialog than the one you described. If that were to occur, the reason is usually the presence of other files, such as Time Machine backups of other Macs.

Possible that option was deleted in OS 10.15.1 or something earlier?

It's possible. I'm not at a Catalina-running Mac right now I can't be sure if that option remains available or not. Personally I don't think it's very useful anyway, and probably only caused unjustified concern. Time Machine's Preferences already shows the time period each backup device contains, whereas often cryptically worded dialog boxes are easily misinterpreted. That misinterpretation sometimes leads people to doing things they shouldn't do... such as manually deleting files using the Finder in a misguided effort to create more space on their own. You can do that, but there are a lot of caveats, and it takes a very very long time. I strongly recommend not doing it.