I have 3 external hard drives attached directly to the rear USB 3.0 ports and one USB 2.0 hub with 2 hard drives attached. About once a week one the directly connected hard drives disappears. I have to pull out and reinsert the USB cable and the hard drive comes back. It has happened on two of my hard drives - Fantom drive and a Seagate drive. Any help. I did uncheck the " put to sleep" to save energy.
Usually it is down to loss of power, especially if they are USB powered drives. If it is a laptop you are connected to then you may be exceeding the allowed voltage from the USB ports. Portable drives use 3 volts, anything lower that this causes drop outs. I hope this helps.