Hi,
I have an extra, spare desktop PC that I would like to know if I could use in the role of an external hard drive for backing up all my files to from my main desktop PC? Both desktop box's have USB sockets and I have an extra USB cable. If I could do this, what would be the best way to do it?

Thank you

I don't like using usb to usb myself but it could be done maybe.

One common way is to allow some remote access to this extra box via network. A ftp server would do for your home. Many commands in linux can be piped to or over ftp. A simple floppy of g4u could back up your system or consider maybe clonezilla or another.

No but it depends on the hardware. Typically PCs only have host ports and therefore can not be used as a client device such as a hard drive or a mouse. In a nutshell USB is a master/slave architecture where by the PC (master) controls communication to whatever is connected. Two hosts can not communicate between each other. In addition you do not want to short out the 5VDC between the to computers. There are OTG USB whereby a port that can act like either master/slave but typically AFAIK only found on embedded devices.

As stated it would easiest to use ethernet and configure your spare computer for network file sharing. There are USB host to host cables but would probably be more expensive then a network adapter.

What operating system is installed on the spare computer, and is it connected to your network?

One option is to set up the computer for Network Attached Storage (NAS): http://en.wikipedia.org/wiki/Network-attached_storage

There are open source options such as FreeNAS.

Another option is to take the hard drive out of the spare computer and place it in an external USB drive "caddy" to convert it to an external USB hard drive (your goal).