[BetterRed]

@jeffdonnelly - I didn't claim its Windows fault, if its anyone's fault its possibly yours.

At some time in the past I suggest you had a calibre library located on a device that no longer exists; the device shown in the Windows error message.

With version 2.85.1, you need check what libraries are configured by clicking the library icon. If you read the last post in the link I gave you will see the OP had a library configured on a non-existent device from the long ago past - removal of that library (via the Calibre Remove library option) fixed the problem.

Possible Explanation: The old calibre server feature could only access one library, the new one can access multiple libraries, by default these libraries will be those configured in the Library Manager.

So, I'm guessing that calibre 3.x checks to make sure all the configured libraries actually exist at start up, by getting Windows to open the library folder - if a path to a library is on a non existent device then Windows throws a device not found message.

BR