How can I downgrade a mid-2010 21.5" iMac from Yosemite to Snow Leopard? 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