how do you rename an external drive in OS10?

While ext HD mounted, select it's icon, hit the enter key which will highlight the name, type in the new name, and hit the enter key again.

Since you're a newcomer to the Mac, see these:

Switching from Windows to Mac OS X,
Basic Tutorials on using a Mac,
Mac 101: Mac Essentials,
Mac OS X keyboard shortcuts,
Anatomy of a Mac,
MacTips,
Switching to Mac Superguide, and
Switching to the Mac: The Missing Manual, Lion Edition.

Additionally, *Texas Mac Man* recommends:

Quick Assist,
Welcome to the Switch To A Mac Guides,
Take Control E-books, and
A guide for switching to a Mac.

Select any drive icon whether internal or external. Press RETURN to activate the name field. Change name to your liking and press RETURN again.