External hard drive back up

What kind of network? Are you using a switch or router? If it's just two computers networked with no router or switch, I'd just attach it to one of the computers, then map that drive on the other. If you're using a switch or router, you can use a network storage adapter that allows you to plug the USB cable into it, then you plug the adapter into the switch or router with an ethernet cable.

This is possible. Once you've mapped the drive on one of the PC's and given the user permissions/right to whatever files or folders are in the back harddrive simply have the user make routine backups with Windows Backup.

Or you can use the Syncronization manager to have the computer make copies of selected files or folders. All you have to do is right click the mapped drive and select synchronize or click the Tools tab and click "Synchronization" and that lets you specfiy what/where you want to synchronize files.

I don't think it would work using a media converter and plugging the harddrive into the switch/router since you can't give it an IP address.