External volume disappeared

We have our office network set up with a range of static IP's. We connect to each other by hitting Command + K to pull up the "connect to server" dialog box and punch in the IP number of the person we want to connect to, put their username and password, and then connect to the drive we need to access (usually external hard drives)

This morning, one of the external hard drives disappeared from everyone except the the computer it's plugged into. The hard drive shows up and is accessible only to that computer. If we connect to that computer, it brings up a list of every volume except that one, even though it's plugged in and functioning properly.

any ideas?