I’ve experienced an interesting USB 3.0 problem just recently that I like to share with you.
On this freshly installed system:
- ASRock H170M Pro4 (with latest BIOS 7.20)
- Core i3-6320 (3.9 GHz, 2C/4T)
- 16 GB RAM
- Win8.1 Pro x64 fully updated
- USB 3.0 with MS in-box driver
- RST 15.2.0.1020 driver + console installed via setup.exe (from Asrock web site)
I connected this 3.5" external hdd case via USB 3.0 to it:
- FANTEC DB-ALU3e-6G
- USB 3.0 with UASP protocol or eSata
- ASM 1153E manually updated to firmware 140509_A1_82_40
- 8 TB HGST 7.200 rpm
to transfer a big number of video files to one of the internal hdd’s (Western Digital Blue WD60EZRZ) of the H170M Pro4 system. The case was instantly recognized and the UASP protocol was enabled (uaspstor.sys).
The file transfer speed started around 150 MB/sec, pretty much what you can expect from the two hdd’s involved. But within one to two minutes, the transfer speed constantly slowed down to a poor 60 MB/sec and remained that low. 
Changing the policy from ‘safe remove’ to ‘performance’ didn’t change this. Some registry hacks found on the internet didn’t change this either.
Then I’ve changed the connection of the external case from USB 3.0 to eSata. The last free sata port on the H170M Pro4 is configured as hot plug in BIOS. Now I got the full transfer speed permanently at 140 to 150 MB/sec! But why not over USB 3.0?
This external case runs perfect at full speed on my Z170 main system (see my signature below), which is basically the same hardware (at least the Sata and USB 3.0 ports) and the same Win8.1 Pro x64 fully patched, same USB 3.0 MS in-box driver, but (differently) the MS in-box sata driver. So, the only obvious difference was the sata driver!
Looking into the RST console of the H170M Pro4 system I discovered that sata link power was enabled by default. Turned that off, reboot and tried again. And FINALLY now permanently with the full transfer speed of around 150 MB/sec over the USB 3.0 port! 