You can get a Laplink USB to USB. It has what amounts to a cross over in the middle (with a chip probably) and the 5v's do not feed each other. A simple USB to USB will cause damsge and you still don't have the resident software.

You have load software on each PC and then the other PC looks like another HD on the system - just the HD mind.

External HD cases and HDD are usually USB 2.x. If one of the USB sockets is 1.1 it doesn't work. I have that situation with my various PC's. I have to remover the HDD and use the IDE connection. Your laptop HDD has a different connector, surely. Even if you were brave enough to remove it.

Laplink have made serial and parallel port cables do what is wanted. I bet you could find them cheap now. And if the PC's are old what OS 's are involved? There are cheapo versions of Laplink products but one slip and you load the wrong driver and - they are useles. I should know. Go with Laplink.