how long will it take to transfer 1gb on a usb 2.0 and a usb 3.0?
A decent SSD connected via USB 3.0, though, could do your 1GB file in 2.5 seconds. OTOH a mediocre USB thumb drive might only sustain about 10 MB/s so would take about 2 minutes. A normal HD can sustain writes of about 150 MB/s, so would saturate USB 2.0 (1GB/40MB/s=25 seconds) but not 3.0 (so 1GB/150MB/s=6–7 seconds).