Have connected a portable disk, ISODRIVE 100 VIA USB to my
system. Previously I am able to locate and access the drive
when I click My Computer. But now, unable to view the drive
from My Computer and under Disk Management, it shows
that the disk is unallocated. Please help as I have lots of
data on the drive. Thank you.

Removing the external drive from your computer improperly can result in data corruption.

Have you always used the "safely remove icon" to disconnect the drive?

Try the drive on another system. XP seems to lose drives occasionally. I suspect it accidently corrupts the Master Boot Record occasionally when it encounters read errors.

Thank you very much Ms OtheHill and Aegis for the advise.
Yes, I have used the Safely Remove Icon to disconnect the
drive and have also try using another system to connect the
drive but still the same. I think the disk is corrupted and
would appreciate it if any one can advise how to retrieve the
data from a corrupted disk. Thank you.

You could try Knoppix. Knoppix is a live version of Linux. Knoppix installs to and runs from one CDR. It may be able to recover files if all that is wrong is a corrrupt file system. Google for Knoppix.