I have had this happen with my PRS 600 several times
Not sure why.
Several thing I have done and not sure which helped as it has been a while.
Turned computer completely off then restarted.
hard reset of PRS 600
Used different port
Maybe had to do both several times.
Unplugged all other USB devices.
Could also be a cable problem.
Also running Windows in safe mode may have helped.
Sorry I cannot remember exact sequence, process that worked.