Originally Posted By: tanstaafl.
Can I use those disks to nuke and repave the unhappy 20" iMac, or is it like Microsoft where the OS is registered to a single computer by CPU serial number and can't be put on another computer?

Are the discs grey in color, or do they have a big X (roman numeral 10) on them? If they are grey disks, they are pretty much only useful on the machine they shipped with. If it's an X, it would also probably work in the 20 inch iMac. The difference is that grey ones are "OEM" discs, often with other licensed software such as iLife and bound to the machine it came with. No serial numbers bind it to the machine, instead the installer simply goes "Is this an iMac 24 inch from year XX?". Discs with the X on them are retail copies and work with any Mac older then the discs themselves (due to driver support).

Originally Posted By: tanstaafl.
Apple takes a different approach with its OS X software for the Mac. It intends to offer a modest new version every year. Installation is a 15-minute, one-click operation, and the price is piddling. For OS X 10.8, Mountain Lion, which came out Wednesday, Apple wants $20 — and you can install one copy on as many Macs as you have, without having to type in serial numbers or deal with copy protection hurdles.

Ok, so with Lion and Mountain Lion (10.7 and 10.8), Apple moved to digital distribution, and automatic whole house licensing. Licensing in the past was 1 computer only, or they sold a 5 pack. It's all been an honor system though.

Anyhow, the digital download changes the situation a bit. You have to have OS X 10.6 Snow Leopard or higher installed to download 10.7 or 10.8. Since the 24 inch iMac already has 10.7, you are covered here. Look for the Mac App Store entry in the Apple menu to buy and download Mountain Lion for $20.

Once it's downloaded, it will pop up the setup screen and want to install. Cancel the install, because when it runs, it will upgrade then delete the installer, thus requiring a second download later. Instead, go and download this program too, ( http://blog.gete.net/lion-diskmaker-us/ ) and run it. It's going to help you create a bootable USB or DVD of the Mountain Lion install, possibly usable on the 20 inch iMac.

Why did I say possibly? Well, the 20 inch iMac may or may not work with 10.8. I'd recommend checking this site below to identify which model 20 inch iMac it is, and seeing if it's new enough for the upgrade: https://www.everymac.com/mac-answers/os-...ntain-lion.html

Also, it would be good to know how much RAM is in both the 20 and 24 inch iMacs. Mountain Lion is a little more memory hungry then older versions of OS X, and it runs best with 4GB or more.