Shared drives no longer network accessible on iMac running Mountain Lion.

Hey all,

I've been searching the community on and off for weeks and have not been able to come to a solution. I have an external hard drive connected to a 2007 iMac running Mountain Lion that has successfully been shared on my network for over two years starting with Snow Leopard. Just recently I noticed the shared drives were no longer showing up on my networked Macs (running anything from Leopard to Mountain Lion). What's odd is the shared drives will connect and show up for the first 5 minutes or so after the iMac is restarted, but will go offline soon after. The iMac shows up in the side bar, but I get the dreaded "Connection Failed" message in Finder when I click "Connect As" after the shared drives first go away. Screen sharing works fine all the time when accessing the iMac. I've tried everything on the iMac from disabling the firewall completely to renaming it on the network to disabling SMB on all shared partitions. Anyone have any suggestions?