iSync has absolutely nothing to do with storage devices. It has to do with synchronizing address book-type data between your Mac and any one of a number of handheld devices (PDAs, Cell Phones, etc.)
"Syncing" your data between the internal HDD of your Mac and your new external HDD is more of a concept than anything else. If you're looking for automation, your best bet is to get a data backup program. Retrospect from Dantz is an established program which you can use to do this; however it's commercial and not exactly dirt cheap (although you do get what you pay for).
Another alternative is Mike Bombich's Carbon Copy Cloner, which will basically make a duplicate copy of one drive onto another, and also has a range of useful features.
Naturally, the absolute cheapest option available to you is to simply drag-copy all of your user data to the hard drive yourself, and then on some kind of periodic basis copy over new or recently-changed data. It isn't automatic, and of course requires that you actually do the keeping-track-of and copying, but it's the tried-and-true method that most people out there (including myself) use.
I'm a newbie to this site and am looking forward to being a member. This is my problem. I recently bought a external hard drive, Maxtor lll. My main goal for purchasing it was to put my iphotos on it so it would free up some space on my internal drive. However, when I take a photo off iphoto on my internal drive, the same thing happens on my external drive, and visa versa. I phoned Maxtor and they said it was a computer problem relating to "sync". I don't know how to work with isync. 