EricGAnderson wrote:
Apparently the MacBook stores snapshots that it will write out to the time machine when it’s reconnected. That was taking up a lot of space. I’m not so sure that’s a practical idea but that’s what apparently happens.
If you read the post I linked previously explaining macOS storage, it did include mention of APFS snapshots.
Time Machine backups allow you to find multiple older copies & revisions of your files. This is extremely useful in case you made a catastrophic edit a week ago. As long as your external drive is large enough, you will still have that older revision of the file available to you in the TM backup. Some backup software will only keep the most recent copy of the files, so if you would be unable to recover the older copy. However, this extra functionality does have a price, it requires more storage on the backup drive.
Why am I mentioning all that? Well, that is the reason for multiple APFS TM snapshots being stored on your main boot drive since you have not been backing up to the external TM backup drive. TM is keeping all those changes in the backup snapshots for you so you still have access to those other revisions of your files in case you make a catastrophic edit, or you accidentally delete a file. Plus it allows TM to later move those snapshots to your TM backup drive when you finally connect the TM drive again.
Usually APFS snapshots are not an issue except when you are deleting a large amount of data and running out of Free storage space. If you have a large number of APFS snapshots and make a lot of changes to existing files, then that can possibly consume more data than it would seem.
You still have about 150 GB of Free storage space so you can still use it normally, but just don't do anything which would use up lots of storage space (video editing, etc.) until you transfer those Time Machine backups to your external drive. Completely running out of Free storage space will stop macOS from working and even prevent you from deleting any files to make more room due to how the APFS file system works (unless Apple has changed it to allow a built-in buffer). You want to make sure you keep at least 20 GB+ of Free storage space at all times for the basic operation of macOS.