Comment 4 for bug 1834735

That sounds like the OS is losing contact with the device and reconnecting. I have hit that occasionally and it usually means a reboot of the device and the PC are needed.

You need to check what is happening in the OS/computer. Calibre needs the device to be mounted properly. If it isn't, then calibre cannot do anything. There can be a problem when calibre starts to do its work and puts some load on the USB connection. I see this, but it =is usually a cable/port/device combination. And I always see an error when the device drops.