I am using Armor ATD 4TB. My Write/read speed is slow (~2mbps to 20mbps). G-Technology promises 135 speed, but I never achieved this speed. Can anyone tell, what could be the reason?
published speed is generally in optimal circumstances.
Your speeds are very low though. Try AJA System test or BlackMagic test to check the speeds.
Also try on another pc to eliminate it is a pc related thing.