Don't sweat it, I make typo's too you know
On actually testing it out instead of relying on my rickety and often blurry memory, it turned out to be -
* 6-52'ish with an avg. around the low 20's (rough guess-timate) transferring to the thumb drive
* 20-90 MB's with an avg. around the low 40's (rough guess-timate) transferring to the Hd
The transfer rate apparently depends on the type of file as well.
However, you are talking about mech. drives attached to a usb enclosed hub, using (I am assuming) sata to usb conversion? In any case, I am thinking I remember a program that would backup a drive that deferred to run in the background, using any dead spots in the computers uptime or low resource allocation threading or some such.
While it would take a great deal of time, in other words, it would be less painful from your point of view since your other computer activity would take precedence.
Now, if that doesn't work for you, since you are talking about a hardrive, you might just pop it out of the usb connector and plant it in your case and direct back it up. Of course, that depends on how confident you are in playing with your hardware, I don't work on macs much and am not as familiar with the intricacies of their insides, but I know apple would love to void the warranty like a shot if you even popped it open (or so they've said in the past).