My Mac Won’t Delete Files I’ve Backed Up to an External Hard Drive

Are you using Time Machine and/or having a problem with it before or after moving those files to the external drive...?

I'm asking, because you may have Time Machine snapshots that are being saved locally.

see > About Time Machine local snapshots - Apple Support

and > Deleting Local Snapshots for Time Machine… - Apple Community

OP wrote " About half an hour after posting this, the .mov files finally cleared out and I now have some space. Still not sure why deleting previously backed up (uncompressed) Logic sessions doesn’t free up space on the computer’s hard drive (it’s been months since I’ve deleted them in some cases). "

This seems to dovetail in Purgeable Space

Although you deleted some large files and emptied the Trash

If we may refer back to

" Amount of space available, which can include both free space and “purgeable space”—or space that macOS can free up when needed by removing files from your computer (you can’t manually remove the files that are designated purgeable, but macOS removes them as space is required) "

The Operating System ( not the user ) would hold onto portions of the deleted files until IT decided additional Empty Space is Needed / Required

Then and only then would some of the Purgeable Space would be moved to Empty Space

Alpha6709 wrote:

The problem started before I began using Time Machine (which is set to manual). It seems to be working normally.

Having Time Machine set to manual, could be causing it to store up large snapshots.

see > Deleting Local Snapshots for Time Machine… - Apple Community

and > About Time Machine local snapshots - Apple Support

Look in the Mac's Trash.

If they are there, then you need to Empty the Trash to free up the space.

However before you do that.

1) test the files on the external drive.

2) backup those files to a second external drive.

Get detailed information about a disk in Disk Utility on Mac

• Amount of space available, which can include both free space and “purgeable space”—or space that macOS can free up when needed by removing files from your computer (you can’t manually remove the files that are designated purgeable, but macOS removes them as space is required)