| Sounds like your username that you are logged into does not have permission to access that drive. On the PC drive that you are trying to access, you'll need to right click, go to properties and check the access/permissioning set up.
Make sure it allows either all users or your specific username, depending on how you have the permissioning set up.
Or depending on how you have set up the network, it's possible you need to login to the other PC, possibly having to enter a valid username (and maybe password) that the other PC recognises, and thereby meeting the permissions of the disk drives on that PC. It should prompt you if it wanted a login though, so probably not... | 
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