You, my dear sir, are an absolute legend! 
After I saw the write cache = 0 from your command, I rebooted the Server, went to the BIOS and saw in the AHCI settings, that write caching was turned off. I enabled it, and ran your command again and write caching was turned on.
I tried a large transfer and tadaaaaa!
Thank you so much! Not getting the full Gigabit transfer rate of ~120MB/sec though, but I’m satisfied with this atm.
SOLVED!
“Perfect, thank you HoneyBadger. I was also dealing with this problem for days. Grateful!”
Luis Gustavo Leal
Make sure you have a UPS as if I understand it correctly, write cache enabled can mean you lose data in the case of power outages.
Pending data in any non-volatile storage - an uncommitted transaction group in RAM, or writes to the HDDs - can potentially be lost. A UPS will prevent supply-side power faults, but doesn’t address things like an internal PSU failure, CPU/motherboard/other critical component, etc. Full protection of all in-flight data usually requires synchronous writes - which in turn usually requires really fast drives and/or a log device.
ZFS can manage drive caches if it knows about them - so as long as the drive is honest about what it does when it receives a cache flush command, it doesn’t introduce any additional risks vs disabled.
