i am talking about the FG which is on another hard-disk.
an i wantto know what can i do if that Hard-disk crash's
how can i still work with the Primary one?
You can still "work" with the Primary file group by taking the files on the crashed disk offline.
However, you cannot make the file online without backup.
I would like to add two more points here:
1) You should have RAID. If a disk crashes, replace it with new one.
2) If you take data file offline, the objects on that file will become inaccessible. So your application may not "work".
Hope this answers your question.