I want to use my laptop as an external storage device with my PC. Is this possible? Is there a way to keep a backup of a file on my laptop hard drive and is there any software that can keep the backup on the laptop in sync with the PC?
First you need to share your laptop drive to the network:
Then you need to map it to the desktop PC:
edit I should note, that it is not totally necessary to map the laptop drive on the desktop, it just makes it a bit easier to access
If you want to use the hard drive directly, you want an enclosure.
If you're looking to do a backup, check out Crashplan. Free version will do it.
If you're looking to use it as "live" storage on a running other machine, you'll need to create a share and map a drive.