mb/s is dat megabit or megabyte per seconds. I assume bytes, and that isn't bad for a USB 2 external hard disk.
Consider a USB 3.0 is 3 times faster. So if your backup now takes 15 minutes it will take 5 minutes then. That's why make an incremental backup during lunch or dinner or when looking at the news on the TV. My file backup is ready before I'm ready, so i feel no urgent need for a faster backup device.
But if your backup takes 1.5 hours it might be worthwhile.
I would like to know what kind of download speed I should be getting using a USB 3.0 WD Passbook external HD. I have a Azus P8Z77-V motherboard that has USB 3.0 connections. The internal HD i'm using as a source drive is a WD black.m My processor is an Intel CORE i5. I am mostly backing up MP3 music files. I'm getting speeds of 24-29mb/s I thing that is slow