How do I wipe El Capitan off my I mac and load snow leopard from the disk?

Insert the DVD, restart with the C or Option key held down, use the Disk Utility on it to erase the internal drive, and install Snow Leopard.

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That is a kernel panic.

They are usually caused by hardware problems.

Why did you think you need to downgrade the operating system?

When El Capitan blocks the Nikon software is there any message provided as to why?

There are SO many reasons to downgrade.

I do not seem to be able to boot from the SL disk either. Inserted disk, tried w/both C and option key - got silver screen w/apple in center and then - no farther... any Idea how to get beyond this point?? Thank you

I don't know about the iMac but with my MacPro 5,1, the first thing I'd try is resetting PRAM.

But in order to run an OSX installation DVD, I did recently have to remove all hard drives from the MacPro and put in just one drive that had been formatted using an earlier OSX. The OSX install apps are fussy about the hard drive(s). Drives that have been formatted in Lion or later can't be reformatted using earlier Disk Utilities. Drives that have been formatted in Yosemite or later may be in a state of "core storage". This can be undone with a terminal insruction "diskutil coreStorage revert ...". Sometimes it is necessary to remove a hard drive and thoroughly wipe it.

A Mac's firmware can prevent running an OSX install app, but if the Mac ran that OSX before, and if it's firmware wasn't updated, the OSX install app can run, provided it finds a suitable drive.

DC

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In most cases, Macs won't be able to run older OS X versions than their shipping versions. What is your iMac model?

The original post by ghenry1999 says "return to Snow Leopard". Maybe this means Snow Leopard ran on that iMac, or maybe not.

Perhaps I've been lucky in avoiding the "in most cases" in Lanny's rule-of thumb. My Mac Pro that shipped with Mountain Lion is running happily on Snow Leopard; my MacBook Pro (non-Retina) that shipped with Yosemite is running happily on Mountain Lion. I think the firmware version is more critical than the OSX with which the Mac ships.

DC

I am on a MacBook Air, A1370 Mid 2011. I bought it as a back up for my Macbook Air A1370, because I need to stay with Snow Leopard (for a lot of reasons), if anything happened to my computer, even with a back up clone of the drive, Time Machine, it would not run on a newer logic board. So I bought the same year and model as mine.

The "new" one was loaded with El Capitain and I have been trying to wipe it and load my clone of my hard drive- but El Capitain is not letting me. Same as OP.

I have an external DVD drive, EC wont load the installer, it wont even see the drive when I hold down option and try to select a startup disk.

It wont let me erase or format.

So I used the back up of the EC drive (yes, I backed up first on a USB drive), it would allow it to be the startup disk, and from the backup USB drive of EC I was able to "erase" the El Capitain hard drive.

But then when I tried to reboot and use the Snow Leopard DVD, or even the El Capitain USB drive, hold down option, it shows NO startup disk, I can only use "Internet Recovery".

I had also tried holding down C at the start of this, trying to get it to install from the Snow Leopard dvd. Holding down C then and now only brings me to an Apple logo.

If its an A1370 logic board, it has to have shipped capable of running Snow Leopard, right? Does El Capitain change the firmware on the logic board? I will try resetting the PRAM...

I don't think installing El Capitan changes the firmware, but whoever installed the El Capitan on the 2011 MacBook Air might also have changed the firmware. Apple issued a 2015 update to that computer's EFI boot firmware. Boot into El Capitan and check System Information for the Boot ROM Version. Compare this to the list in Apple's support page.

If you must roll back your firmware to an earlier Snow Leopard tolerant version, how do you find the old installer and will it allow roll back? Apple's "Firmware Restoration" CDs "cannot be used to return an Intel-based Macintosh computer's firmware to a previous version if a successful update has already been performed." This puts you in hackland, or in search of a 2011 MacBook Air with its original firmware.

DC

Wow, I have been trying since last night to post....

these boards went down as I tried... ok so we're back...

I wrote a good reply but apple.com runs a script that crashes my browser...

I will try to write it again... fast

Thank you DC- I managed to get it back to 10.7 using Internet Recovery (for anyone trying, use Command R). Evidently Internet Recovery has Apple check what your computer shipped with and lets you go back that far. I happen to have a 10.7 back up of my father's computer (upgraded to ML against his wishes, so I put it back to SL, but that was 2012, and his computer is likely as old as mine, 6 months older than this new one).

So now it's on 10.7 and I checked the firmware as you suggested, the "new" one is MBA41.0077.B12 and my old one is MBA31.0061.B01 from About EFI and SMC firmware updates for Intel-based Mac computers - Apple Support

except that my old one is .B01 as I tried to write here, but that page has it .B07

So this is firmware on the logic board, not the hard drive?

I have the original install USB "chip" for my Air, and if there were discs I'll have them as well. I'd be happy to take a trip to hackland, if i could figure out what to do. I am hanging on to what you said about having a MBP that shipped with 10.7 that is happily running 10.6...