It could be that when you attach your external drive to the laptop it is allocated a different drive letter from the one it has when attached to your main computer. For example, if you have two internal drives on the main system they will probable be called C: and D:, and the external drive will be E:; however if you have only one internal drive on the laptop it will be called C: and when you connect the external drive it will become D:.
The Archers - give it a rest already!
i7 Quad core 2.66 15GB ram, Windows-7 64-bit, Nuendo-5.5.6 64-bit, Wavelab-6, 2x MR816CSX, RME FireFace UFX, SmartAV Tango, MOTU Express 128, UAD-2 Quad, Lexicon PCM-91, Lexicon MPX-500, TC-Fireworx, Halion-5, Kontakt-5, Roland V-Drums TD-20X
Omnisphere, Trillian + loads of real instruments.