Can't see my external HD

I bought an external HD last week. I connected it, and could access
it. That night I clicked on Remove Safely. Next day - and ever since -
I can´t access it through my laptop with Vista OS, though I can on a
PC with W2K.

(By the way, last week I also had problems connecting to my network.
But with all firewalls switched off, I can get through. I still haven
´t managed to find a firewall-on access, but it's clear where the
problem lies. Thanks to all those who offered advice.)

Jon