I have this issue like once a week. Here’s how I fix it… use at your own risk.
Go into a terminal and type ‘cd /Volumes’ then press enter.
Type ‘ls -al’ and press enter. This will give you a list of the available Volumes.
On my Mac, I see two listings for my external SSD, “Pebbles” and “Pebbles 1”. It’s like Mac OS doesn’t see the SSD for a time, or doesn’t unmount it and creates a new mount point called “Pebbles 1”. In my case, I have to remove “Pebbles” and rename “Pebbles 1” to “Pebbles”
That solves the problem for me. I am doing something very destructive, so please be careful. When I first diagnosed this problem I renamed “Pebbles” “Pebbles_temp” rather than delete it. Def try renaming first.
Hope this helps!