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E13. The Very Bad Key Drive

The following problem was a joint effort. I don’t know the original author. The original version, involving a poisoned keg of wine, was passed on to me by Arshag Hashian of Northeastern via Sandy Blank. My nephew, Michael Ash, modified the statement of the problem to make it politically correct. My sister, Arlene Ash, improved the exposition of my solution.

Here it is:

You have five expendable computers and 240 key drives. Exactly one of the drives has a very bad problem. Any computer that has mounted the bad drive during the previous day will be destroyed when the cron system maintenance runs on the computer at midnight.

You need to use the data on the 239 good drives on a nonexpendable computer in 48 hours and so you can have only two rounds of testing. How can you determine which is the bad drive?

For the solution, click here.