When it comes time to replace a disk – such as when a disk goes bad in an external hot-swappable disk cage – it can be catastrophic to remove the wrong disk (to say the least). Assuming that you have “read” capabilities on this drive, how do you identify the disk associated with the one you know is bad (which you’ve identified from the operating system)?

Well, in a UNIX-based environment, you can use dd. How?

Make the disk light on the disk glow by entering this command:

dd if=/dev/baddsk of=/dev/null

This will read the entire disk, starting at the beginning, and send the output to…. nowhere! It does, however, make the disk light light up.

In system administration, it is never wrong to double-check. So: execute the command, and check which disk has lit up. Then stop the command and watch to see that the light goes out (or is no longer solidly lit). Restarting the command should then light up the disk. So now you know.