Re: Cannot share a drive for some reason
 

Does your main pc have a logon password set?
If not create a new user (maybe an admin just for testing, alternatively set permissions for this user on the share) give it a password, try mapping a share from the other computer but using the new user account (mainmachine\user) details.

You could also try disable passwords for accessing shares by doing the following:
Right Click Start > Select Control Panel > Select Network and Sharing Center (View by: Small Icons) > Select Change Advanced Sharing Settings > Scroll Down to All Networks and Expand > Select Turn Off Password Protected Sharing (Under Password Protected Sharing) > Click Save Changes

Re: Cannot share a drive for some reason
 

No Password is set, Password protected sharing is off.

I made sure all settings are the same on all systems and still the same.

Re: Cannot share a drive for some reason
 

Buy a USB 3.0 hard drive, plug it in and see if that works :D

Re: Cannot share a drive for some reason
 

Click Start > type "services.msc" (without the quotes), check to see if the Server service is disabled or not started. Enable if disabled and set to automatic, click Start.

As a final test, create an admin user on your main machine with a password.
From one of the other machines open a dos prompt and type the following:

net use * \\MainmachineName\ShareName /User:Mainmachinename\Newadminuser /Password:*

Try the IP address of the main pc as well, alternatively use the Map Network Drive function in Explorer.

If that does not connect then you either have a firewall issue, a network issue or something other than permissions, is there a way you can copy the contents of the drive to another place, and then format it fresh and try again.