USB connection
 

Silly question, I suspect! If I remove the HD from any of my out-of-use computers and insert it in my external drive caddy, it appears in the in-use computer drive listing and I can access all its data files. All the external drive caddy is providing is appropriate power supplies and a USB path. Sooooooooo... is it possible to make a USB connection between two computers, one just switched on and the main one active. In other words, can a scondary computer be used as an external drive caddy via USB connection and, if so what cable pin-outs (or what cable type) would be required?
TIA

You are describing a USB-USB bridged cable - these certainly exist and can be had from ebay / amazon etc. If you do want to go down this route, bear in mind that USB2 is relatively slow - slower than simply connecting the PCs together over a wired Gb LAN - but that USB3 is up to 5Gb, provided that both devices have a USB3 port.

Both computers would need to have an operating system installed and booted, however, compatible with the chosen USB bridge cable, and with any required drivers installed.

USB Crossover cable
 

Search for a USB crossover cable. As Saab says, not too fast but does the job, Software inc. just plug in and play. Mine opens a file manager window on each computer and you can copy files across. I've never tried actually opening a file from the remote computer though.
They are fairly cheap , 10 - 20ish, so it's a low risk if it doesn't suit you.

Here you go, but it will not be fast:
https://www.ebay.co.uk/itm/USB-PC-To...4AAOSwg0BZzzgp

It's been a feature on Macs for many years. Initially using a Firewire connection but these days requires Thunderbolt/USB C cables. Never heard of it using USB2/3 connections though.

YES. If you can run an operating system on the file server computer.

Windows has had built in file sharing over networks since about 1991. Windows for Workgroups 3.11.

So you can just use an ethernet cable or wireless networking and map (mount) the remote drives.

Linux also can publish or mount windows files using Samba.

You share the folder (or whole drive) on the PC you want to use as the file server and then map (mount) that remote drive on to the client.

For a novice there are various permissions and other holes to fall down. There will be loads of articles on t'interwebby.

It looks like you may be able to do IP (networking) over USB too. Not read in detail.

Or search for
[file sharing over usb windows to windows]
Good luck.

Gosh! - thank you very much to all contributors. Very reassuring, eye-opening and informative1:ok::D Off to the Bay, it is!!

I said -
"It looks like you may be able to do IP (networking) over USB too. Not read in detail."

The references I saw suggesting that this would work referred to Windows Phone. It won't work with a basic cable PC<->PC. You need electronics in between. This is what the special cables provide.

You could use two USB-Ethernet adaptors and an Ethernet cable (crossover cable may be needed).

https://www.howtogeek.com/190835/how...e-file-server/

Turn it into a NAS.