and what is the exact spec. of your hard drive(s), like model number? did I understand good that your problem lies in the fact that english alphabet contains (only) 25 letters and there are no available letters for your two extra hard drives?
did you try to connect some other hard drives via that unit - like newer drives and try to "mount" them properly?
yesterday I bouth another sata/ide to usb adapter. This one is branded and guess what - the letter is attached to the disk. However, I have a lot of HDD's which I tried to mount. When I connect some (old) models the XP does not attach drive letter to them - even with the new adapter this does not work. In my opinion the solution to this problem lies in the HDD drivers and adapter itself.