Ubuntu doesn't "see" external USB Hard Disk
Okay here's what worked... (Here's a video explanation)
Firstly let's check if Ubuntu detects the device:
Disconnect your device then open a terminal and input ls /dev/ | grep sd and Enter. Now connect the device and input ls /dev/ | grep sd again and Enter. Now you should have two outputs. Compare them to each other.
You should find an extra line (or two) in the second output. Keep that extra line somewhere; we'll use it later.
If you can't find a difference then I don't think my solution will work for you but you can try. Probably you have a hardware problem.
(Here's the second part of the video demonstrating what's below)
Now input sudo fdisk -l in a terminal. You should see a large output.
Wait until the "prompt" (the line that looks like user@host:~$) appears again. It might take up to 5 minutes.
When the prompt appears again now you can now "mount" the device with this command:
sudo mount /dev/sdxn /mnt
Replace sdxn by the extra line you found when we did the first step.
(example: sudo mount /dev/sdb1 /mnt)
And you're done!
Summary: To mount the hard disk again later:
sudo fdisk -l- Wait
sudo mount /dev/sdxn /mnt
You can look here to learn about detecting devices. Detect and mount devices
if the device is recognized, then you can try mounting the device. #mount /dev/sdb1 /home/user/someFolder
if you get errors, post them here. if you have LVM problems, you might find this helpful: http://pissedoffadmins.com/os/mount-unknown-filesystem-type-lvm2_member.html