Adding a 2nd HDD

Buy a portable second HD. Transfer your chosen files to it. Check that they are safely transferred. Then clear what you can from the internal HD.

The advantage of a portable, say 1Tb, is that you can store it (and its valuable contents) away from the machine, indeed, away from the house and therefore protect the contents against fire, theft, lightning strikes and whatever. I use such an HD; my entire system is on it and I top up every couple of months or so.

WD have a range of such portable HDs and I'm sure there are many others. Ideally it should be about 2 or 3 times the capacity of your internal HD. And continue to use Time Machine. What arrangements do you have for that?

Yes, the iTunes files and iPhoto are the obvious targets. You could also consider buying a big capacity external HD and running your computer directly from that.

I don't think it would be worth considering buying a new Mac if the current one is working well. But if you did, you must first check that your apps will function with the totally different system that Mountain Lion (Mavericks) uses.

You can only add an extra one by removing the optical drive & using an adaptor to hold another HD in its place.

See http://www.ifixit.com/Device/iMac_Intel_24%22_EMC_2134_and_2211 for instructions on that & the standard replacement.

Hello again, dsloan,

Always comes over so impersonal, the user name, but never mind.

That 90GB sounds good and much better than your initial forecasts. I do hope all works well for you; thank you for the star.