I have a USB HD connected using SATA to USB connection to the a PC with XP, before it used to be recognised fine and assigned a drive letter but since some malware issues with my PC, I had to repair the XP installation, the hard drive is not being assigned a drive letter so I can't access it. It is detected as USB mass storage however and I have looked in Hard drives in computer managment too. I have also connected the hard drive to another PC and it is accessible.
This happens if the OS is trying to assign it a letter which is taken. Go into Computer Manager and see if you've got the drive without the letter. Then, reassign the letters how you wish.
This happens quite a lot to me when I use a dozen memory sticks in various combinations.