How long would it take to transfer 1TB over USB 2.0?

USB 2.0 has a signaling rate of 480 Mbit/s. The same article says that typical real-world throughput is "about two thirds of the maximum theoretical bulk data transfer rate of 53.248 MB/s." If my math is correct, and it probably isn't, that suggests that the best time you could hope to achieve is about 8.2 hours for 1 TB, assuming that the USB connection is the biggest bottleneck.

From experience, I know USB 2.0 copies about 10Mb/sec on average (on my system).

So that would be
1TB == 1048576 Mb
1048576 / 10 ==> +/- 104857 secs
104857 / 60 ==> +/- 1747 mins
1747 / 60 ==> +/- 29 hours

So a full day and 5 hours.

Note that I use TeraCopy as the default copy handler of my windows (otherwise I never get the 10Mb/sec average over usb).

Given the variations of I/O handling by the operating system and the natural delay of starting and stopping copying (many vs few files) you are realistically looking at approx 120 Mbit/s or 15 MB/s (from my experience)

Theoretical values: 1 TB @ 480 MBit/s = approx 4.6 hrs

Realistic values: 1 TB @ 120 MBit/s (or 15 MB/s) = approx 18.5 hrs

I have seen about 4-5x faster performance with ESata vs. USB2.0 using the same external hard drive. I have a WD 1.5TB Essentials Drive, which I would back up using USB, but it was painful, taking about 4-5hrs per 100G, and running about 8MB/sec avg. I pulled the HD out of the plastic case, removed the USB to SATA board, and plugged the same HD into the SATA port from the mother board via an ESATA cable, and I can now backup 100G in less than an hour, and 250G in about 2.5hr using NovaBackup S/W. All I can say is that 2-3hrs is not that bad for a lot of data (running about 28-32MB/sec w/ESATA). Both are well/far below theoretical, but the comparison between the two is what counts.