Won't mount in 'extFS for Mac'

Trying to mount an 'extFS' external drive (we'll call it 'old' drive) so I can clone it to another one ('new' drive) as the original has had issues recently.
As soon as I plug in the Old drive extFS for Mac starts 'repairing it' but I don't see anything happening.
When I go to mount the old drive it just says 'processing' and shows this for a while now.
Here's what Disk Utility told me when I tried to run 'first aid' on it:
Attachment 852What are my options here?