meaning of disk offline/online

Question

What exactly does it mean to place a disk offline or online (re diskpart command)? Is this the same thing as mount/unmount?

Answer 1

Essentially yes.

http://technet.microsoft.com/en-us/library/cc766465(v=ws.10).aspx

Unless you have dynamic volumes, then there are extra considerations and stuff. If you don't know whether you have dynamic volumes or not, then you don't. You have basic partitions.