As per the subject:

Can a disk that's been set to offline in disk management be infected by a virus?

(As far as I know, the disk is not SEEN in PC as a mapped letter drive e.g. D:/
however, it can easily be turned on in disk management)

Current situation:
Host OS running windows 10
Guest OS running windows 8.1 (can be upgraded to 10 if relevant) - Through a windows to go USB

Host OS disks have been deactivated in disk management when booted into Guest OS
Guest OS cannot access Host disks unless I go to disk management and turn the drives online
If guest OS is infected with a virus (testing suspicious files), can the Host OS offline/unmounted disks be infected?

(This process involves no physical detaching of the drives as it is a tedious solution if deemed unnecessary)
(Please also do not SUGGEST SIMPLY not to TEST this at all. I would like to know HYPOTHETICALLY what level of risk this involves)

I have been informed that Evil Fantasy or SuperDave may be able to help with this question.

Thank youMore information about that topic here.