Having issues with external USB drive configuration

[mr.goodtime]

I have a Seagate 320Gb external drive (Mac OSX Extended - Journaled) plugged into my Airport Extreme USB port. I can see and work with the files fine, so no problem there. I work for a CADD firm and several of us work on the same project at the same time and it could be disastrous for two of us to be working on the exact same file without knowing it. Ever since I reformatted the external drive, I’m not getting warnings that someone else is using that file. I’m used to getting a warning dialog offering to open up a read-only file. If someone could explain to me how this works and whether this is an issue with my computer, the network or the external drive, that would be great. I appreciate any help.