Time Machine: the oldest backups are NOT deleted when your disk becomes full

It is not possible to say exactly what is causing this without knowing more about your backup. I think this could happen if you have multiple machines backed up to the drive. I think that Time Machine will only delete old backups of the machine you are currently backing up.

That seems like a reasonable explanation given that you only have 7 months of backups of a 250 GB drive onto a 3 TB backup drive. That math doesn’t add up. Either you have backups from other machines on this backup drive or you have some other problem with it.

Normally I would agree with Barney-15E. Just erase the backup drive and start over. But if you have multiple machines backed up to this disk, then that would delete backups of all of those machines. I think it is time for a new backup disk.

I don't use Time Machine in part because of the massive space needs for backups. But you can use the Time Machine application to delete whole backups as well as just files. This is the only correct way to delete backups and files from a backup. Were I in your situation I would erase the drive and start over. However, you can keep the entire backup drive and replace it with a new drive.

I use Carbon Copy Cloner and one backup drive equal in size to my internal drive. I do incremental backups but overwrite old files rather than keep them. Therefore, my backup drive never fills up. However, I also do not have old copies of files which is the big feature of Time Machine. It's definitely a trade-off one must make.

Unfortunately, Apple doesn't have a Time Machine Guide. However, you will find a lot of information at the KB by going to support.apple.com and searching for "Time Machine." Apple has a lot of up-to-date articles on all things Time Machine.

One page of a guide for macOS. I mean a full guide made from the couple of dozen support links in the KB. Something that covers all the possible Time Machine topics, errors, problems, etc. Pondini's old site was a pretty good guide at the time, but it's now out of date.

Kappy wrote:

Unfortunately, Apple doesn't have a Time Machine Guide.

Really? What about https://support.apple.com/guide/mac-help/mh35860/mac?

You don't quite understand how that works:

Time Machine deletes older files if they have been deleted from the source when it needs space on the backup drive for a new incremental backup. Time Machine "thins" it's backups; hourly backups over 24 hours old, except the first of the day; those "daily" backups over 30 days old, except the first of the week. The weeklies are kept as long as there's room.

How long a backup file remains depends on how long it was on your Mac before you deleted it, assuming you do at least one backup per day. If it was there for at least 24 hours, it will be kept for at least a month. If it was there for at least a week, it will be kept as long as there's room. By default, Time Machine backs up hourly. That cannot be changed in Time Machine. There are third-party utilities that will modify the backup interval such as Time Machine Editor.

The Time Capsule sparse bundle grows in size as needed, but doesn't shrink. Thus, from the user's viewpoint of the Time Capsule, it appears that no space has been freed, although there may be space in the sparse bundle.

Once Time Machine finds it cannot free up enough space for a new backup it reports the disk is full. You can either erase the backup drive and start over or get a larger drive.

Just erase the drive and start a new backup, but you likely need a larger backup drive. Starting a new backup might last a little while before you run into the same problem.

There is no omission. Old backups are backups of files that are no longer stored on your Mac. Removing "old" files from the backup while they still exist on your Mac wouldn't be much of a backup.

Time Machine does not make a copy of every file on every backup. It only makes a new copy if the file has changed.