Troubleshooting Error communicating with device?

[keeperofthebooks]

When I go to transfer books in Calibre to my Kindle it has a pop-up that says: error communicating with device, this is the error codes I get. Am I doing something wrong?
(21, 'GetDiskFreeSpace', 'The device is not ready.')

Traceback (most recent call last):
File "site-packages\calibre\gui2\device.py", line 82, in run
File "site-packages\calibre\gui2\device.py", line 412, in _upload_books
File "site-packages\calibre\devices\usbms\driver.py", line 256, in upload_books
File "site-packages\calibre\devices\usbms\device.py", line 1010, in _sanity_check
File "site-packages\calibre\devices\usbms\device.py", line 175, in free_space
File "site-packages\calibre\devices\usbms\device.py", line 138, in _windows_space
error: (21, 'GetDiskFreeSpace', 'The device is not ready.')

[geekmaster]

That "device is not ready" error can be caused by a number of things. It could be a bad USB cable, or a problem with the USB drivers on your host PC, or a low battery on your kindle, or a problem with firmware or software installed on your kindle. Or perhaps your calibre installation is damaged. Or, it could even be an antivirus or security suite or firewall program that is preventing communication between calibre and your kindle.

You could try restarting your kindle (charging its battery first, if needed). You could try rebooting your host PC. You could try a different host PC, or a different USB cable.

If none of that works, try temporarily disabling antivirus and security software. If that does not help, you could try uninstalling and reinstalling calibre.