How do I delete most of my time machine backups

EDIT ---- Deleting backups is likely to gain less space than you might expect, since files that do not change do not get copied over and over. So don't expect to get a lot space back.

You can do this with tmutil in the Terminal. Be CAREFUL. Issuing the wrong command can result in data loss.

First, it is easy to check which backups exist:

tmutil listbackups

This will produce a list of the backups, looking something like:

For example, if your TM drive is called "TM", and your mac is called "mymac", a backup done yesterday at noon will appear as

/Volumes/TM/Backups.backupdb/mymac/2018-04-08-120000

Now for the dangerous part. It requires the sudo command to get admin privileges, so it can only be executed in an admin account (if yours is the only account on your mac, it is automatically an admin account).

To delete one or more TM backup, you use tmutil with the delete verb.

For example:

sudo tmutil delete /Volumes/TM/Backups.backupdb/mymac/2018-04-08-120000

You will be asked to enter your password. While typing nothing will appear onscreen. Then after pressing enter the command is executed.

You can delete more than one backup at a time, but CAUTION --- This is dangerous!!!

For example, to delete all backups from August last year, you can type

sudo tmutil delete /Volumes/TM/Backups.backupdb/mymac/2017-08-*

PROBLEM IS if you make a mistake you may delete a lot more or, something different, than intended.

USE AT YOUR OWN RISK

See this macOS Sierra: Remove a backed-up item from your backup disk

It was designed to work on it's own, but doesn't always. My wife backs up her iMac up to my Synology. I have a 8 drive/20TB array for her backups. It fills up constantly, then Time Machine just stops working because the target disk is full. So in this case, I need to manually purge older backups (as much as I reject the idea, because it isn't a good idea.)

This problem isn't new, her previous 3 iMac's and my previous SAN storage arrays, TM would always fill the volume and then just stop working.

Why delete old backups?

Time Machine was designed to work all on it's own. When the backup drive fills up it will start deleting the oldest backups as it requires more space.