18:17 May 20, 2011 

David explained it himself: "a drive may be physically present, but not [logically] available"
I do believe that this refers to a hot swap drive as you can insert it with the server running. I am confident that the backup drive could be removed with the system still running without harm, however, the author probably does not consider the backup complete before the system has been shut down (and all data safely written to the backup disk). Maybe this procedure is good practice for server backups to hard disks?

06:50 May 20, 2011 

Although surely a system that allows hot-swapping/hot plugging wouldn't require the computer to be shut down in order to remove a drive, would it?

18:48 May 19, 2011 

"mount" might indeed not be the best choice here, since the context suggests that a drive is physically removed and a new one inserted/plugged in. If this replacement is done without shutting down the system, is it called hot-swap(ping)/hot plugging, and yes, there are RAID systems that allow hot-swap. See http://en.wikipedia.org/wiki/Hot_swapping

12:37 May 19, 2011 

Although "mount" is a commonly used verb in connection with hard disks/drives, it has a very distinct meaning. The way I understand it, the instructions call for one hard disk to be physically removed from the RAID array and replaced by another one, which "mount" does not require, as a drive may be physically present, but not available, for any length of time if it is not mounted,

08:18 May 19, 2011 

by the instructions for perfrming subsequent backups:

Fahren sie das System herunter, entnehmen sie wieder eine Platte, fahren das System wieder hoch und bauen dann die beim letzten Mal ausgebaute Platte wieder ein.