Downloading old macOS from a newer Mac to install on an external hard drive I have an older MacBook Pro that had some HDD issues.  Now the Mac doesn't have a working OS and I need to install MacOS.  The newest version that it can use is Mac OS Mountain Lion.  So I borrowed a friends new MacBook, logged into the Mac App Store with my Apple ID, and the bought it.  

But now I can't download it because when I click download, it says this version of MacOS is not compatiable with the newer MacBook.

Which is fine, but I'm not trying to install it on the new Mac, I just want to copy the install over to an external hard drive so that I can then use it to install macOS on the older mac.

Any ideas?
What model (and year) is the older mac? 

Hold down cmd-opt-r on restart on your new Mac. This will boot into MacOS Internet Recovery where you will be able to install the OS that shipped on the new Mac (if its compatible - thats why I need the model and year). From there you may be able to install or upgrade your MacOS version.