I take it that you are running windows xp on the problem machine?
It sounds to me that windows is not mounting the drive correctly. In most versions of linux you have to do this manually but windows should detect and mount the drive automatically.
I am not sure of a work around for this, you could try going into Contol Panel/Administrative Tools and manually mounting the drive there.
If this is the problem then and it keeps occuring then I would think it would mean a reinstall.
You could also check to make sure all of your bios settings are as they should be, but I don't think the problem will be there.
If you could let me know the affected operating system it would be a help. Meanwhile I will see if I can recreate the fault on one of my machines.