Is OS on Logic Board or HD?

I'm getting ready to order new battery for my 1st Gen Original iPod. I'm wondering if I should replace the hard drive while I'm in there. It looks simple enough--just plug and play.

But before I do, I'm wondering if there's some primitive (no longer supported) version of iOS or PodOS or whatever on the hard drive? I'm afraid that if I swap it out, iTunes will no longer support it.