Windows XP is disabling the network shared status of our external hard
drive if the drive is not present when Windows is restarted.  We use
this external hard drive for off-site backup and therefore it is not
always available when Windows must be restarted.

How can we force Windows XP Professional to remember that, when
present, a certain folder on an external hard drive should be shared
on the network even when the computer is sometimes rebooted without
the external hard drive present?

I have the same problem. After a restart, you need to reactivate the
external hd. It is a pain to do this manually and I haven't figure out
a way to do it with in windows. Next best thing I did was wrote my own
program that run at start up to automate the process. But I'm pretty
sure there's a way to do this without resulting to 3rd party programs.