To safely detach a volume, umount the mount point first:
sudo umount /mnt/workspace
Confirm it is gone with df -h or ls
Now you can detach the volume within Horizon
Go to Project > Compute > Volumes and pull down the Volume Menu to Manage Attachments
Click on Detach Volume
The volume is detached, but not deleted. All data is safe.
The volume can be reattached to the previous VM or attached to another VM (e.g., another piece of the workflow)