I'm not any guru, but:

Fran, I think this is because your external drives are not assigned with drive letters other
than automaticly when plug into laptop. The first you plug in will adapt the first letter not
taken by onboard devices etc.
You can give each device it's own driveletter as well as name who will remain when plug/unplug.

I don't have any English language PC here at the moment, so ask anyone confirm my translation
before do anything.
Be sure both devices are plugged in, and that you know what driveletters they got before you
continue.
Rightclick "My computer", select Manage and then Manage Drives. Then you can rightclick the
drive you want to give another letter.
This way your index know where to find the content next time you plug in.

Sorry for my bad memory on what comes up on english PC, but be careful and don't change C: or
other onboard letters already taken!

Hope this help, but get it confirmed by someone before do it.

Cheers
GJ