Premium Amiga 500 boot selector from http://www.sordain.ie - I bought one of these for my Amiga 500 so I could use and external floppy with a Gotek drive as the boot drive.  It works fine for that, but one thing to note is that if you have the external drive selected as DF0: (Boot drive) it will not see the internal drive at all, not even as DF1: .  The only way to see both drives is to have internal as DF0: and external as DF1: . It isn't a huge deal, but would be nice to be able to swap the drives.

P.S. - Never swap drives while the computer is on.  You have a good chance of screwing up your CIA chip.