Because disk space is so precious on my MacBook, I have to store larger hard drive images on an external drive. When I want to run Parallels without the external drive attached, I have to edit the PC definition to disable any external drive images before I can start up. Furthermore, I have to think ahead when I'm deciding whether to shut down the VM or just to suspend it. It would be most awesome if you could configure an IDE channel as a "removable" drive, with an image selection pulldown similar to what's available for the CD. Then I could still suspend Parallels, without having to worry about whether it will be convenient/possible to first attach my external hard drive.
Here's my thought... I'm assuming that the external drive is either USB or FW based. If it's of the former, when PW gets the USB support included, it may be possible for it to 'lock onto' the external drive (if you used VirtualPC before, you know exactly what I'm talking about... if you don't, it's essentially the VM gaining exclusive usage of the USB device). I'm not sure how exactly the USB implementation works with PW, but if it's done like this your problem will be solved.
But I don't want Parallels to see my external drive - only to use disk images which happen to be stored on the external drive. The drive is primarily for the Mac's benefit, not XP's. :^)
