One of my external hard drives suddenly was no more accessible - I had left it connected after shutdown, and when restarting some windows update caused a fauilure when trying to read the external hard disc.
I was lucky to have a friend who had a colleague in his IT business that had the proper tools to copy the contents of the EHD, re-format the drive and copy the data back on, so all was still there. He recommended to disconnect all external drives after shutdown for this reason. I never had a problem accessing data even when I had not used them for months.
I use a powered multi USB hub - most computers do not have enough power to drive more than one USB device at at time.