External (USB) drive no longer shared after reconnect

Have successfully setup sharing through my router -- internal drives and
one USB external drive (Western Digital Passport). The network consists of 2
computers running Vista Home Premium and 1 running W2K-Pro; and a Linksys
wireless router.

After applying the required 'shares' and 'permissions' to the USB external
drive (which is connected to one of the Vista systems), I was able to see and
modify all files on the external; which is what I wanted. When done, I
disconnected the USB drive (using the "Safely Remove Hardware" utility),
since I routinely take that drive to anothe site.

The next day I reconnected the USB external to the same Vista system as
before. But although the internal drives could still be seen across the
network, the USB drive could not. In fact, when I check Properties /
Sharing, it now shows as "Not Shared". For what it's worth, that drive
displays as the same drive letter as it did originally.

This may not be a Vista issue. It may be due to the drive being USB
connected. Just don't know. Haven't been able to find a reference to this
issue in the Microsoft Community groups or generally on the internet. Any
ideas?
Thanks much,
JohnDavid