I have a HP M380N Media Center PC running Windows XP and I'm trying to hook up a Buffalo Dual 250 GB DriveStation and a MyBook Essential 3TB drive. Should Windows XP be able to detect and assgn drive letters to all three drives and have them show in MyComputer? So far only the MyBook and one of the Buffalo drives shows up.
Yes you should see them all.
Try swapping power and USB cables in the buffalo to see if the missing drive appears. check the BIOS and see if it reports the drive (Not all BIOS will report USB drives) Try using a different USB port on the computer, try uninstalling all the USB root hubs in the device manager.
How to use Disk Management
To start Disk Management:
Log on as administrator or as a member of the Administrators group.
Click Start, click Run, type compmgmt.msc, and then click OK.
In the console tree, click Disk Management. The Disk Management window appears. Your disks and volumes appear in a graphical view and list view. To customize how you view your disks and volumes in the upper and lower panes of the window, point to Top or Bottom on the View menu, and then click the view that you want to use.