Ok, it obviously hasn't been a very clever question and I understand both
opinions stated during the last hours.
But just imagine you're new to this stuff and haven't heard yet of any
differences between sequential and random access media. Would you guess you
have to look into the device class section to find out?
I like those messages! They remember me of the time when all this stuff was
loaded with questionmarks for me, too. 

I'd like to welcome Hon Wai Wong in a list where it is not common to get
such harsh replies. Usually when a question occurs and I think the answer is
not of common interest I try to help with a personal mail. Now the
discussion already has taken up more bandwith than necessary.
************************
Hon Wai Wong,
usually all backups target to a tape drive when you use a conventional
backup system like BackupExec or ARCserve. With ADSM you have much added
functionality, like you can backup more than one client at once or you can
follow an "incremental forever" strategy.
To get faster backups and faster access to your data ADSM not only uses
tapes but also disk space. Small files first get backed up to a disk pool
(primary storage pool) on your ADSM server and then get migrated to tapes,
as the primary pool gets filled up.
Main difference between those two pools is the media type: On the primary
pool "random access media" gets used, that is a harddisk or a file on a
harddisk. The pool for migration uses "sequential access media", that
usually are tapes (readable sequentially from start to end).

Now you might guess why you cannot add a tape to a diskpool...

Just one more hint for using this list:
You can download ALL messages from the past from the listserv server. Read
the message you got back when you subscribed to the list. You'll find a
description on how to download a list of available files.
Then just browse and get loads of questions and corresponding answers, try
to grep for some keywords and find out you're not the first one asking.

Good luck,
regards,
Michael