errormacos
I purchased a second hand iMac with Mavericks Developer installed already, seen as I am not a developer and other things don't work, I purchased a copy of Mountain Lion but when tring to run the installer it gives me this message
"alert mac os x 10.7 cannot be installed on this computer"
I want to keep all user profiles and software intact so I don't want to wipe the hard drive (although many other posts say when you get that message wiping the hard drive still doesn't solve it)
any ideas?
I just did this yesterday, so it's still fresh in my mind. Make sure that when you restart your computer, you hold Cmd-R to enter recovery mode. Inside that, choose "Disk Utility" and there you can erase your Mac OSX partition.
If you do not do that and simply select "Reinstall Mac OS X", then it will not erase your drive, but just overwrite the parts that need to be overwritten.
Note that you will not be able to erase the Lion recovery partition this way. All options to modify this partition are disabled in Disk Utility. There is really no need to wipe this, and erasing and formatting the main partition should be sufficient.
The Time Machine backup should do a pretty good job keeping all of your documents and most of your settings and should be backwards-compatible*, but I recommend making a complete, bootable backup of your previous system so that you're guaranteed to be able to restore your machine to exactly as it was before updating.
I'm a fan of Carbon Copy Cloner (free trial) for making the backup. Back up to a clean disk or partition using CCC and tell it to copy everything. It will also help you clone your Lion Recovery HD onto the backup disk.
Once the backup is done be sure to try booting from it to make sure everything worked.
*Should be, but I haven't personally tested.
Best Answer
Verify that your iMac can run the older version of Mac OS X. You can see what versions can be run by searching for your specific model of iMac at EveryMac
Sometimes I have seen that even if a Mac was shipped with an older version of Mac OS it is difficult to downgrade due to firmware upgrades that make the system incompatible with older releases. I haven't personally experienced this sort of issue in maybe 10 years.