yes I have the same issue too, I found that the conflict was down to conflicting assigned drive letters,
To confirm this issue, boot your computer with your reader attached, Run Calibre, If the reader is not detected then unplug, wait for the reader to finish its content check then plug the reader back into USB.
If this resolves your issue you would need to move some of your assigned drives letters around in disk manager.
Although to be honest, I still find issues with connecting my player to Calibre, it seems to be MUCH more finicky than previous 5.x.x.x versions. Even at home on my personal computer with perfect drives assigned sometimes calibre just decides to act the arse.