HiFirst error looks like USB issue. Never had it, but some users do, using an original cable and/or a USB 2.0 port and/or a USB HUB seems to help.
As for the second issue with "unknown", run this command, write here the output:
fastboot getvar product
The output is:
C:\adb>fastboot getvar product
Finished. Total time: 0.016s
I am using the original Xiaomi USB cable. My PC is quite new, and its USB ports are probably 3.0. I will try it with a USB HUB.