Thanks for the response!
I went into Windows Explorer and found only a few folders and two files, one of which was related to Calibre. I deleted absolutely everything I didn't need.
When I restarted Calibre, I got the same error:
calibre, version 0.8.45
ERROR: Error: Error communicating with device
[Errno 22] invalid mode ('wb') or filename: u'F:\\metadata.calibre'
Traceback (most recent call last):
File "site-packages\calibre\gui2\device.py", line 82, in run
File "site-packages\calibre\gui2\device.py", line 339, in _books
File "site-packages\calibre\devices\kindle\driver.py", line 329, in books
File "site-packages\calibre\devices\usbms\driver.py", line 243, in books
File "site-packages\calibre\devices\usbms\driver.py", line 387, in sync_booklists
File "site-packages\calibre\devices\usbms\driver.py", line 385, in write_prefix
IOError: [Errno 22] invalid mode ('wb') or filename: u'F:\\metadata.calibre'