Wow this seems to be a huge pickle...I wouldn't suggest taking it out of the case but it may be a option if not sorted out soon
Does it make any weird noises or vibrations when the drive first powers up? My friend has a really wierd external drive that sounds so sick it's not funny. Also, check whether any other Pen/USB drives work, as it may be a USB port issue. Are you plugging the USB cable in before or after the Ex-HDD is switched on? if possible, switch it on and wait for it to spin up and initialize completely before plugging it in (could be up to 30 seconds)
of course it is very well possible it is simply dead and cannot read itself? One noise to look for is a sound that sounds like it is making small grinds every second or so, I have had many drives go this way where it cannot read itself so it continually tries to read that small file, and I would find leaving it to grind over night managed to open the drive and every click I made started the grinding all over again.
If it is a silent casing this may be why it seems so mysterious
Kurtis