Tosh - I may be totally out of line here with regard to your problem, but the following information may be of assistance to someone if not your good self:
Several years ago, I was given an iPod with about 3.5 Gb of memory. I downloaded iTunes and after a bit of a struggle - well I am using XP on the other type of computer, I managed to make the program do what it said on the tin. With loads of tunes on my iPod, and a decent set of headphones, it worked beautifully. I never ipdated my music collection on it for a number of years and then lost its computer lead, but in the meantime iTunes had updated itself several times. Finally, I decided that some more music was needed so I purchased a new interface lead, iPod to computer at Apple's humungeous prices. On connecting iPod to computer, the battery charged up, but that was all it did. There was no way I could get the current version of iTunes to interface with my 3 year old iPod and revise its music collection despite many, many attempts.
I therefore determined that the iPod was bust, the new lead was faulty, or that the latest version of iTunes would not interface with my iPod.
and thus revisited my local Apple store from which I acquired my new lead.
They were extremely helpful despite me making it clear that I didn't want to buy a new Mac! My iPod and lead were tested and their current copy of iTunes promptly uploaded a bit of fresh music onto my iPod. Then they considered my dilemma and I was advised that my virus protection (Avast) was interfering with iTunes. I was thus instructed to download a new copy of my virus protector and a new copy of iTunes. Then, after disconnecring my machine from t'internet, instructed to delete my virus protection and iTunes. Then I was to re-install my latest copy of iTunes, then do likewise with my new copy of Avast. I did so, reconnected to t'internet, updated everything that wanted updating and then, with baited breath, connected up my old iPod. Guess what - iTunes worked beautifully and my music collection on the iPod has now been updated as required!
Now, I appreciate that it isn't the same with a Mac, but the order in which software us installed may also be significant. Furthermore, your local Apple store may well prove to be as helpful as mine was. Full marks to them for this, completely free, service.
P.P.