This unit has a very old F/W?!
in any case the disk is mounted already as a normal disk. to reconfigure as raid Please instruct user to remove "disconnect" the new disk since it is already mounted as normal and wait for a while and re-insert the disk .
Next, I suggest
1) backup your data
2) update the firmware to the latest version ( Optional but strongly commandeered:)
follow the manual instruction: with latest version :
*When the replacement hard drive is recognized, the status LED will flash red. The TeraStation will start rebuilding the RAID array automatically.*
with older version
*Drive -> click the RAID array and select the new drive* next push rebuild raid. See manuals for details.
see this faq for more information
http://au.faq.buffalo-global.com/app/answers/detail/a_id/15184/kw/rebuild%20raid
if you can not rebuild raid please post raid screenshot status full screen after you select raid.