SSD unable to delete

Thanks Barney

I have left the encrypted SSD mounted all night, and there is absolutely no change in the free space indicated: 4GB. So there is no garbage collection operating. Is it possible the 74 snapshots are taking up the space which should be available? However there is no access or way of removing them that I can see.

Since no further files can be added to the SSD, it is essentially useless unless I save the entire 1TB of files and then reformat it, which I am loath to do unless absolutely necessary. Is this a common problem with SSD?

No, DU does not give any way to access the snapshots.

Thanks everyone for your comments.

I have now checked that all my snapshots were created by Carbon Copy Cloner. The Safety Net feature was not causing the problem. The backup snapshots are stored on the destination volume; the older ones can be safely deleted if storage becomes a problem, though auto-culling is configurable. My problem was that I had also set up the source volume for snapshots as well, and these had accumulated without my realising it. Since the source volume is automatically backed up regularly, these source backups aren't necessary, and deleting them restored my SSD capacity.