Resposta do kolypto em ServerFault :
When you "safely remove" a USB HDD - it stops spinning. I believe there's a USB command to turn the device off (sleep mode, actually). So the thing you need is how to turn the device back on.
The application Safelyremove has a command-line tool that is capable of returning the device back: "Returning just stopped device back!".
It is shareware, but it works :)