OK, so there’s two questions.
1. Can I mirror from an internal drive to a USB drive?
Technically yes. Performance will be horrible so I wouldn’t ever recommend it.
2. Can I share the drive with computer computers?
Sure, using Windows file sharing.
Go pick up a cheap NAS from Best Buy, Costco, Fry’s, etc. or use Windows Live Mesh to sync the folder onto all the computers you want to share it with.