set password for a folder

[obscurant1st]

Is there any application available or is there any means by which I can prevent access to a particular directory in my External Harddisk regardless of the operating system in which the external may plugged in.
My operating system is Ubtuntu and ubuntu only. but I used to give this hd to some other people. I dont know which OS they may use and I dont want them to go through my private files.

[Oak37]

I'm not aware of a way of enabling permissions that translate across different operating systems.

One way would be to encrypt the directory or the individual files utilising PGP keys and Seahorse, there's a guide here.

Alternatively you might consider truecrypt or something similar.