Its not possible what you want, by USB, because there is communication on both sides and disk needs an agent to communicate with. Only one.
Because of that exists switches, which have a controller to allow a USB mouse/keyboard to be used in >=2 PCs (and you need to switch between both, not using in both computers at the same time).
Imagine too the problems of update at the same time the MBR or the FAT table from two computers. That will be a huge BOOM!
Then you have the following options:
- Use a switch and change the availability from one PC to other.
- Share the mapped unit in Windows/Unix/Mac by network.
- If you are using windows, for example, Remote Desktop is a nice and smart way to access to the master's PC HDD from other PC.
Obviously 2nd and 3rd setup requires that one PC is all the time on, else you won't be able to access.
If that doesn't happen, to access at the same time from both PCs, then you are in the exact place to get a switch (around 30$).
And the last option is a NAS, which is a special "network enclosure", more similar to a server, and with an IP direction...and so on. But maybe thats a bit expensive for you if you plan to use that disk just a bit casual, not all the time working with it. Else, take a look and see if it matches to your budget. I find them a bit expensive for a normal home scenario.