Unmount and Detach

You can use the umount command (note the spelling of the umount command) to unmount your device from its mount point (here /pvol). E.g.

$ sudo umount /pvol

You can use the Nectar dashboard to detach a Volume from an instance. Navigate to the Volumes page, and use the action menu for the particular Volume to show the Manage Attachments dialog. (Alternatively you can use the action menu of the instance on the Compute / Instances page)

Detach then Delete!
Volumes must be detached before deletion.