The message means that the file M:\metadata.calibre is read-only for some reason. Calibre needs to be able to write to that file.
The file can become read-only for several reasons, most of which have to do with problems on the card. You should check the card. Make sure calibre is not running, plug in the device, right-click on M: (assuming it is still M
, choose 'Properties', then 'Tools', then 'Check the drive for errors'. Check the box that tells Windows to fix the errors.
After that finishes, try calibre again. If the error still happens, stop calibre, then delete the file M:\metadata.calibre (and only that file).