IMac – How to downgrade from El Capitan to Snow Leopard

Any time you have problems starting a mac with an installer - do check these three articles:

• Finding your Mac OS X version and build information
• Mac OS X versions (builds) for computers
• http://support.apple.com/manuals/# - enter your serial number on the page or in place of the # in the URL to determine the exact model of your mac.

In your case - you probably have a MacBook Pro (Early 2011) which will boot from (Mac OS X v10.6.6 Build 10J567) or later. You might see the term "retail disk" which means it has drivers for all Macs that can run that level of OS - this is different from the disks that ship with macs - you almost never can use say a MacBook Air restore disk on a MacBook Pro - they just have the drivers for that one model.

Normally, you could just buy a retail copy but the last 10.6 retail copy I've seen documented is 10.6.3 and is too old. You'll need to order a replacement disk through AppleCare online or via phone (or find someone in possession of your MacBook Pro's install media for Snow Leopard)

Most Apple Retail stores are set up to image your mac from the factory builds through the genius bar and since your mac is so new, I can't imagine the seller you bought it wouldn't bend over backwards to make sure you got a copy of the appropriate disk (they might even have a service desk that could assist you)

There is a huge list of things which can be wrong here:

1. It sounds like you don't have the original install DVDs that came with the MBP (MacBook Pro). If you have an external DVD drive available to you then I would suggest running the Apple Hardware Test version 3A171 that came on the original install discs with that model.

2. • You managed to clone the version of Snow Leopard DVD that you have to a USB drive. I assume you didn't use the faulty DVD drive to do this?

   • Also you need a retail version of Snow Leopard (barring the original install discs), if you have the packaging, then you could see if it has the green dot that says 10.6.3 on the front of the box. You cannot use the original release of the retail Snow Leopard install DVD as your computer probably does not support even a minor version of the OS older than it shipped with (which was 10.6.3). If you're not sure, when you boot up from the install USB check on the "About This Mac..." menu item under the apple, or put in the machine you used to clone the drive and navigate to it and look at the read me files.

   • Also, note that a DVD that came with another computer, unless it's the same model as yours, will not work to install. So, if you took the install DVD from even the first release cycle of a Mid 2010 Mac Mini that had 10.6.3 on it, it still will not install on your machine.

3. Just this week I opened up a machine and it a) had a bad Hard Disk Drive, b) had a dirty/faulty Optical Disc Drive (aka Superdrive, dvd/cd drive, etc.) and that was possibly ejecting discs from a stuck trackpad clicker c) would not boot up from a USB Installer of OS X Lion until I removed the faulty Hard Disk Drive!

   So, you should use the "Disk Utility" Program under the "Utilities…" menu in the installer to run the "Verify Disk" action on the internal Hard Disk of the MacBook Pro (aka MBP). Because, it just might be that the Hard Disk you are trying to reinstall on is bad!

4. You might also have a bad, corrupted USB drive, I would use whatever other machine is at your disposal and run verify disk on the USB drive as well. If none of the other things seem to work then I would even erase and Reformat the USB drive as HFS+, then clone the Snow Leopard install DVD back onto it.

   Also, I have had a scratched Install DVD that of course wouldn't work, this can translate to files not coming over to your USB drive properly.

5. If this is a backup copy of the Snow Leopard DVD there is a much greater chance of something going wrong with DVD. If memory serves correct, you actually need a DVD-R DL, known as a "Dual Layer" disc to make a backup copy of Snow Leopard. It can't be a DVD+R DL (notice the "+" symbol") known as a "Double Layer" disc. I wouldn't burn it at higher than 4x, and use a known good burner, maybe run a laser lens cleaner on it first if possible.

Note: If you're still having trouble after checking these things out, please let us know what tools you have at your disposal to continue working on this, for example, another computer, an external disc drive, a bare drive dock, tools/ability/confidence to open up the machine and work inside, etc.)