Problem: USB hard dks disconnect

[Trinket]

I had a similar problem with my Seagate 1TB USB3.0 disk.
I came to the conclusion that the problem was with the USB3.0 adapter that came with it or the power supply wasn't giving it enough amperage and it was draining it from the Raspy.
I disassembled the disk and replaced the power supply+usb3.0 adapter to an spare one i had from another HDD that had usb2.0 and now it has been working for over a week where before it struggled to reach an hour!
Maybe you could try cutting off the power cable on the usb, or attach it to a usb hub that doesn't allow current feedback.

Also i noticed something different, before when I tried to check HDD standby state with "hdparm -C /dev/sda" I always got "unknown" now, with the new adapter, i can check the current status always.