I'd suggest the following. Get rid of the Property, then:
Code:
beforeRunningCopy()
-- Put your own code here that should execute just before running
the copy.
tell application "Finder"
if not (exists the disk "G5Backup") then
try
do shell script "diskutil mount `disktool -l | grep 'G5Backup' | sed 's/.*\\(disk[0-9s]*\\).*/\\1/'`"
end try
repeat while not (exists the disk "G5Backup")
delay 1
end repeat
end if
end tell
end beforeRunningCopy