I suspect I'm not the first person to have this problem, but I don't have the first clue how I would search for it.

Basically, I didn't read the sharing instructions (classic stupidity) and assumed that if I downloaded the software onto my flatmates' machines and logged myself in on them everyone would be able to have full access to the drive I have plugged into my device. However, it hasn't worked out like that at all - the last guy whose machine I logged in on has full access and the rest of us do not. I have tried logging out and back in on my machine, and uninstalling and reinstalling the software on my computer, but to no avail. 

How do I undo the mess I've made?!

Thanks in advance for any help...