That was a problem many users experienced a few months ago after Windows 10's Anniversary Update -- which has been resolved by now thanks to a fix issued my Microsoft, though. If this is not available to you, one workaround that seemed to work at the time was to format the reader to FAT32 using either Windows' standard format tool or software such as MiniTool Partition Wizard: after that you would need to set it up again with your account and everything, but it should connect to your PC normally again. If you don't want to lose your books and settings, you could copy all of the drive's contents by connecting it to a computer running a former version of Windows (or even 10 with no Anniversary Update) or by running a Linux live distribution on your own PC, and then copying everything back onto the reader after you have finished setting it up.
This was the thread where the issue was discussed,
this a version of the workaround.