"USB2 HDD for back-ups, but the transfer rate is very slow"
transfer rate for usb 2 480Mbits/second.
You can identify whether your PC has Hi-Speed USB2 or not relatively easy. Open Device Manager and expand the Universal Serial Bus section. There should be an "Enhanced" USB host controller present.
To answer your other question network the two PCs and connect with a crossover cable
For some basic ideas:
In Windows HELP, click Index, then type:
Below that double click "Networking"
Tells all about how to connect computers,
and setting up your network.
Run the Wizard on the Host computer, make
the floppy, and run the floppy on the
Client computer. If each of two computers
has an Ethernet card, you do not need a hub or
router. Just an RJ-45 Crossover Cable.