What are you trying to do with the above? Those commands make no
sense; it looks like you're trying to mount an already-mounted volume,
and you're referring to it in a way that won't work whether it's
mounted or not.
If the disk is already mounted, then you don't want to "mount"
it. In the usual UNIX terminology, you want to "unmount" it (why
we use that when the usual English opposite for "mount" is "dismount"
is anyone's guess), and the UNIX command to do it is "umount" (guess
the first "n" made it too long to type). However, there is no
"unmount volume" in AppleScript; the corresponding idiom from the Mac
side of the house is "eject"ing, by analogy with floppies and CDs.
If you're talking about hard disk partitions, using the UNIX commands
mount and umount via "do shell script" is probably the *easiest* way to
script things; I'm not prepared to claim that it's the only way.
But if you're talking about re-mounting a local filesystem via loopback
NFS or local AFP or something like that, there's no reason you can't
use "mount volume" and "eject disk". But the argument to
"mount volume" has to be the fully-qualified URL, even for local shares
(e.g.
afp://localhost/VolumeName), and the argument for "eject disk"
has to be whatever name shows up in the output of the "list disks"
command.