There are two type of external hard drives, the ones that have their own power supply and those who get the power through the USB port itself.
I believe this error is the result of the particular USB port not being able to supply the required power to the external hard drive (and hence the computer does not recognise the drive anymore which can be seen by looking at the "open devices" ).
If you use an externally powered hard drive that will probably resolve the issue.
Alternatively using a different USB port could also solve the problem as some ports might have different maximum power outputs.
I hope this helps,
Chris