Charlie - thanks for the word of caution.
My understanding was that low air pressure was only an issue for Microdrives, as the platters are so close together. For a normal hard disk, this is not a problem. Certainly, I have exchanged emails with people who have used partable hard disks and laptop computers at over 5000m without problems.
In any case, I will let you all know how I get on. I'm off on Saturday...
Julian