Answer #1: Go to tigerdirect.com and search for hard drive enclosures. USB ones work quite well. They're fairly inexpensive and easy to setup. Make sure you get one that is compatible with the hard drive interface (i.e. - SATA or IDE)
Answer #2: If you have a home network, you can keep your "external" drive plugged into one computer and share it. To accomplish this, go to My Computer, right-click on the drive, select Sharing and Security. Click share this, then click the Permissions button. Select Everyone and click OK, then click ok in the sharing box. This way, you'll be able to make changes to the data stored on your external. If you do not have a home network, the best way of course is to plug it into the machine you want to use it on. To access it from another computer, go to your Network connections, then in the left pane click View Workgroup computers. Open the computer that has the hard drive attached, and it should be visible in the next window. Cheers!