Who can say?
Remove the drives from the enclosure...attach each one at a time to a working system...and see if you can read any data on either/both.
Every user who goes to a USB enclosure...just increases the possibly items which can go wrong...with no return that I can see, other than the fact that the drive more easily moved from one system to another.
Your enclosure, your USB ports, the drives themselves...those are the obvious suspects. You can eliminate 2 in one step by removing the drives and attaching each directly to the motherboard of a working system.
FWIW: The file system could also be troubled...the drive itself may be good. But since the file system is what allows Windows to detect the drive...the result would be inability to read any data on the file system.
Just thought of this...if you formatted the two drives to appear as one...and Windows only reflects 1 700GB drive...all is as it should be, IMO. I'm not sure why you did that...but my suggestion for determining if each drive works properly still goes.
Edited by hamluis, 01 March 2011 - 05:28 PM.