Were you running them in some type of RAID setup in XP?
Is there an additional driver you might need to install? I would think the unit was responsible for showing both drives.
My eSata runs off my second Controller. The external drive did not work well until I installed an additional driver for that controller, or at least that is how it appears.
On one of the FAQs it does say if both drives are not seen there is a problem with the unit. Is that problem a driver? I do not see a Win 7 driver download, if I am looking at the correct unit.
Maybe take the drives out and try one at a time.