We have a Hopper 3; with U508 1/30/18 software. We also have 3 external EHDS connected. USB 3.0 to the rear 3.0 port (d1), 2.0 to the rear 2.0 port (d2) and a 3.0 connected to the front 2.0 port (d3). All of the hard drives can be seen and transfer recordings to and from the Hopper 3. Occasionally, the hopper does not recognize the drives. The day can start off with all drives seen and d2 or d3 will drop out and later in the day the drive or drives will return. There seem to be no reason the action. All drives have independent power supplies and compatible cables; USB2 and USB3. Because of the length, there are extension cables for each drive.
When the 3.0 drive (d1) is connected to the rear 3.0 port; the Hopper to EHD transfer appears to be faster. The only constant is the USB 3.0 drive (d1); it is always present. A reset, the usual, will bring back the recognition of all drives.
Suggestion, It may be nice to have USB3 ports x 3.
Whenever a directv drummer attempts a comparison, I just smile and walk away.
Are you resetting the receiver or the EHDs in order for them to be seen again? Some EHDs have a sleep mode that is enabled by default and this needs to be changed through a PC and it does sound like this may be what is going on, so I'd check that too