(Preamble) I bought a new Macbook Air when I realised the battery on my Macbook 2009 was swelling (just the excuse I needed). I thought I'd get a new battery for the white Macbook and set it up for my Luddite wife.

I started with the original installer discs, 10.4, and worked my way up through the updates. I was tempted to leave it at Snow Leopard, because I've always like that system, but since my wife will mostly be using the laptop for the internet, I figured I'd need the latest browsers. I had been running Yosemite on the Macbook.

I had a DVD of Mountain Lion, thinking I could to straight to Yosemite, but no. I can't go to Sierra, either. Searching for an installer, I read a tip that you should go to your App Store purchases history and download installers from there. I did, and found installers for Mavericks, El Capitan and Sierra, but nothing for Yosemite. (/preamble)

So why have Apple abandoned Yosemite? The 10.10 installer app on my hard drive says it is corrupted when I try to run it and says go the the App Store. Not available. The combo updater for 10.10.5 will not run over my current Yosemite system, saying it is not compatible.

Was Yosemite too good? My Macbook will not run Sierra, but it looks El Capitan is as far as I can take my upgrades, even though I know it will run Yosemite.

Too bad.

I can't really help, but I can tell you that I've got Yosemite in my App Store purchase tab, but I haven't been able to get Sierra to that tab no matter what I've tried.

Take a look at your /Applications/Utilities/Disk Utility sidebar; your Yosemite installer may show up there, and if it does, you should be able to run Repair Disk on it.

Worst case scenario, eBay has got plenty of listings for installers of all flavors of OS X...if you'd care to trust one of them.