You can buy (at the hardware) flexible 'sticks' of 1-2' long with small magnets at the ends. They're used to help retrieve nuts & bolts from car engines when dropped while you're fixing stuff under the hood.

Gosh, from the subject title, I thought you wanted to nail down a disposal. Why would anyone want to nail a disposal to something? ;-)

Ah yes, a nail went down inside. That makes more sense. Use either an extensible magnetic pickup tool, or one of those grabbers with a set of claws that extends from a flexible shaft. I keep both of them around for just those times when something falls into an inconvenient place.

John

Thank you for posting this problem. It is not unusual at all for this to happen. Always cover all plumbing drains if working above them.