WinXP Home odd boot behaviour when disconnecting an obsolete drive

I recently installed a new hard drive, did a clean WinXP Home install onto
it, and had the previous installation on my old hard drive as a second boot
option. I then partition copied the old hard installation onto a second
partition on the new hard disk to create a third boot option. So when I
boot I have options as follows:

1 - Clean WinXP home (new disk)
2 - Old WinXP home (old disk)
3 - Old WinXP home (new disk)

All three boot properly, and 2 and 3 definitely boot from the different
disks!

When I disconnect the old hard drive, boot option 1 is fine, obviously 2
fails, and 3 seems to boot OK right up until the Windows splash screen that
I normally get a second or so before the user account log in. At this point
it stops and never progresses to Windows log in so I can't access the old
installation (on the new disk).

Reconnecting the old hard drive allows me all three boot options again and
they all work fine. The old disk has a new home waiting for it, but until I
figure out how I can disconnect it and still have access to my main WinXP
installation, it can't go anywhere.

Does anyone have any ideas?

Thanks

Andrew

PS - All copies of WinXP Home are up to date according to Windows Update.