I'm not recognized as Administrator!

I am the only user of my computer which is not part of a network, but a stand-alone set-up. I have an external hard drive, but suddenly can't access it.

I keep getting one of two messages: 1) I don't have Administrator privileges, and 2) E:\ cannot be found; parameter error.

How can I correct these? I have gone to Start>Settings>Control Panel>User Accounts and am definitely listed as Administrator. And I absolutely don't understand the parameter error thing.

Thanks.