Linux only: allow NVMe destination drives

ON LINUX PLATFORM ONLY

Allow NVMe destination drives as long as they are not mounted
except under /media

This does increase the risk of someone accidently selecting an
internal NVMe drive that is not mounted under the current OS
(Linux) but do is being used by the other operating system
(e.g. Windows) on dual boot systems.
But is deemed to be an acceptable risk.

Ref #158