Can't erase, mount or unmount external hard drive
My late 2008 Macbook failed to boot properly the other day, giving me a grey screen with the Apple logo, the rotating gear, and a progress bar, before shutting down again when the progress bar reached about 15%. The same thing happened when trying to boot in safe mode, but I was able to boot into Recovery Mode. Following suggestions on here, I restored the contents of the computer's hard drive to a formatted external hard drive using the Disk Utility with the intention of then reinstalling OSX, then restoring back from the external HD. It seems though that this has transferred whatever problem the Macbook's internal HD had to the external one.
Using a separate working desktop computer, I plugged in the external HD, which recognises it in the Disk Utility, but can't mount any volumes. I have tried to mount the volumes both from the Disk Utility and Terminal. I can't repair the external HD because it can't be mounted, however nor can I erase the HD because - Disk Utility and Terminal (even using 'force') claim - at least one volume can't be *unmounted*. I ran lsof in the terminal on the separate partitions of the external HD, and nothing came up. I then tried to unmount each volume of the disk through Terminal, just to be told that they were all 'already unmounted'.
So... I can't repair, and am I advised to erase. I can't erase because I can't unmount some volume, and I can't unmount because they're all already unmounted.
I simply want to format that external HD - it has nothing on it that needs to be saved. If I can then somehow save a few files from the Macbook, that'd be nice, but not the end of the world as I have copies of most of its contents. If you need any more details/logs/whatever, I can post them.
-----------
Details:
Late 2008 13" Macbook, OSX Mavericks, 250GB internal HD (broken laptop)
LaCie 500GB external HD (broken external HD)
Mid 2011 27" iMac, OSX Mavericks 10.9.3, 1TB (working desktop)
