I had Yosemite on my old iMac (21.5", mid-2010), then decided to try Snow Leopard on it. (It came installed with Lion, but this model did ship with Snow Leopard previously.)

Basically, I tried doing what's suggested in this thread:

How can I downgrade an iMac12,1 from Lion to Snow Leopard?

I put the iMac into Target Disk Mode, then connected to it with my MacBook running SL. I formatted the iMac HD, then ran the DVD install from my MacBook (an old 13" black MacBook), using the iMac HD as the target.

This is where things went awry.

I rebooted the MacBook with the Option held down, but Startup Manager would not show me the iMac target disk. My only option was the MacBook HD.

Is this because the MacBook does not have the right drivers for the iMac?

I have also tried to install the Mac OS X 10.6.8 Combo Update onto the iMac from the MacBook, but that installer will not accept the iMac HD (with the SL install started on it) as a target.

Any idea how I can complete this install of SL?

Your iMac was delivered with 10.6.6 (somewhat later with 10.6.7). When you have the original (grey) discs that came with it you can use disc 1 to install the Snow Leopard version (not the disc of another machine). Then update it to 10.6.8

I had the same question weeks ago and I format the SSD removing Yosemite and reinstall Mavericks via Internet boot.

If you install via Internet I guess the computer will download the drivers.

Have a look at this site: https://discussions.apple.com/thread/6604300

Your iMac (21.5", mid-2010) should have been shipped with 10.6.3. The latest 13" black MacBook was shipped with 10.5.2.

The installer DVD from your MacBook will not install Snow Leopard. Additionally your iMac will not work with 10.5.2. It doesn't contain the necessary drivers.

Normally you should be able to use the original installer DVD shipped with your iMac and then update to 10.6.8.

Alternatively you may clone your iBook to the (target-mode) iMac drive with Disk Utility, CarbonCopy or SuperDuper!.

I'm not that terribly familiar with target disk mode and whether or not you can perform an installation with it, however you should be able to install OS X Snow Leopard just fine if you take the conventional approach of formatting the hard drive OS X currently resides on and installing Snow Leopard with install media.

  • No as the install media will have an earlier version than came with the iMac so does not have the correct drives – Mark May 11 '15 at 19:26

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