All the above information provided by Fivestring is valid.

If the Mac Band in a Box worked before it should still work now unless you dropped it which can do some catastrophic damage.

You have not told us what iMac or Operating System.

However, try the following:

If you have any external HD's ( other than the Mac Band HD ) disconnect them whereby only your Mac computer is in control plus the external Mac Band HD plugged in..

Leave your Mac Band in a Box HD plugged in.

Reboot.

Launch Apple's Disk Utility and do a permissions repair of your Mac computer.

Reboot and your Mac Band HD should show up.