Except for the floppy, any permanent storage device will take precedence over any removable storage device. HD's, even externals, are permanent storage. You can try going into device manager and assigning the drive letters manually. It might work, but you could also get some unexpected results. I would start by changing the externals to unused letters like X and Z, then reboot. If the CD's then show the desired letters, change the externals to G and H and reboot again.
