Comment # 2 on bug 1131337 from Andreas F�rber

Since it works unreliably first thing to check would be the power supply.
Usually for USB disks an external hub is recommended.
In your link I see nothing about connecting 5V pins, please explain in detail.

Also, is this with Factory or hardware:boot:staging u-boot-rpi3? (Which
version?)
Since probably no one else on this bug has that HAT you'll need to help
pinpoint.