I am getting an unknown file transfer error (Flashing TWRP)
The FAILED (data transfer failure (Unknown error)) error can happen for a few reason. It happens when there is issues with communication transferring data. To trouble shoot the cause start by: 


Trying different usb ports
Trying different usb cables 
Update fastboot.exe 
Download another .img file and verify the md5.
Double check the Bootloader is unlock