I had the same problem a while ago.

Here is my solution:

Open your Computer and right click your External Hard Drive.
Go to Properties.
Click Security.
Click Edit under a quote "To change permissions, click Edit."
Click Add.
Type in "everyone" in the white box.
Then check in the box "Full Control".
Press OK.

This makes the hard drive usable by everyone (all users and different
computers)
It should do it, as i tried it with my friend.
cheers


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aleqsie