OP, can you clairify what kind of machine you have? The slowest iMac G5 out there is a 1.6Ghz.
As long as you are sure that your friend knows what he's doing it shouldn't be a problem. It's not hard really, but someone who doesn't know how to do it might just as well screw it up.
Just be careful and it will be OK.
And I hope you have learned your lesson now. Making backups of important data is very very important. Because sooner or later the hard drive will die and everything will be lost. If you keep it on two drives, chances are much better that you'll have it around longer.
Whats an even better solution in my opinion is to get an account at some internet backup service.
Unless you have large amounts of data to back up that's a really good option, because even if your equipment gets stolen or your house burns down all your personal photos and other things will be kept safely at a server somewhere.
Is there one you recommend?