You don’t tell us much, but “mount volume theNameOrURL of volume to mount as unicode text” works for me. See the Scripting Additions dictionary. Parameters include: “as user name”, “in AppleTalk zone” (not required if URL is used), “on server”, and “with password”.
set diskName to "Backup HD" as Unicode text
set deviceName to do shell script "diskutil list | grep \"" & diskName & "\" | awk '{print $6}'"
do shell script "diskutil mountDisk /dev/" & deviceName
or this…
set theDisk to "Backup HD" as Unicode text
do shell script "diskutil mount `diskutil list | awk '/ " & theDisk & " / {print $NF}'`"