I've,just,bought,a,new,Mac,with,mavericks,,but,I,don't,like,mavericks,,how,may,I,downgrade,to,Mountain,Lion,10.8.5?

You can't.

You cannot install or run an OS X version lower than what is, originally, installed on your Mac.

Your only option maybe to purchase a download code for OS X Mountain Lion here,

http://store.apple.com/us/product/D6377Z/A/os-x-mountain-lion

Purchase a high quality USB 3.0 or Thunderbolt hard drive, formatted as an OS X exetnded (journaled) drive with a GUID partition scheme and install OS X 10.8 Mountain Lion to the external drive and boot and run your new iMac from this external drive.

This is the only optioh to run an older OS X version on a new Mac.

I have a 2009 iMac that the lowest OS X version I can run from the iMac's internal hard drive is OS X 10.6.1.

I have an external drive with OS X 10.5.8 and can get my iMac to boot and run this older OS X version from the ex eternal hard drive.

Here is a good source for quality external hard drives.

http://eshop.macsales.com/item/Lacie/9000353/

http://eshop.macsales.com/item/Lacie/9000303/

Your only other option is to return your new iMac within 14 days for a full refund and look for older iMac, either new or used that came with OS X Mountain Lion as its original, preinstalled OS X version.

Good Luck!

Your method still involves using the computer to boot, which may cause some problems.

Install OSX Restrictions

You posted:

or run an OS X version lower than what is, originally, installed on your Mac.

Booting off an external does exactly that it appears to me.

You can't downgrade since it came with Mavericks. Purchase a redemption code using MichelPM's link, download the installer from the App Store after you get it, and then set it up within a virtual machine such as Parallels or VirtualBox.

Well,

After you posted that link, I remembered that the external drive containing OS X 10.5.8 Leopard was cloned using CarbonCopy Cloner from an earlier Mac.

It still is able to be booted from my 2009 iMac when I select it from the Startup Disk preferences icon.

OS X 10.5.8 is wicked fast on my iMac with 16 GBs of RAM. It never, ever ran this fast on my earlier PowerMac G4.

MichelPM wrote:

Well,

After you posted that link, I remembered that the external drive containing OS X 10.5.8 Leopard was cloned using CarbonCopy Cloner from an earlier Mac.

It still is able to be booted from my 2009 iMac when I select it from the Startup Disk preferences icon.

OS X 10.5.8 is wicked fast on my iMac with 16 GBs of RAM. It never, ever ran this fast on my earlier PowerMac G4.

A 2009 iMac shipped with Leopard, so it is bootable by Leopard.

Barney-15E wrote:

MichelPM wrote:

Well,

After you posted that link, I remembered that the external drive containing OS X 10.5.8 Leopard was cloned using CarbonCopy Cloner from an earlier Mac.

It still is able to be booted from my 2009 iMac when I select it from the Startup Disk preferences icon.

OS X 10.5.8 is wicked fast on my iMac with 16 GBs of RAM. It never, ever ran this fast on my earlier PowerMac G4.

A 2009 iMac shipped with Leopard, so it is bootable by Leopard.

My 27 inch screen, 2009 iMac was bought used with the original OS X 10.6.1 Snow Leopard version installed.

I had to order and purchase a replacement OS restore disc for my 2009 iMac from Apple and the disc has OS X 10.6.1 as the OS installer written on this disc.

I have the 3.06 Ghz CPU model.

You can confirm this through EveryMac.com OR MacTracker.

Very often, especially then, Macs could boot with the previous OS as the bundling of the new OS was not required by the newer hardware. The newer OS was just the most current OS, not a requirement of the new hardware.

There are Macs that require a specific update version of an OS and won't boot from the base OS (10.x.0) version. This happened a lot during the 10.6 era as new hardware was introduced during the reign of 10.6. The newest version of the retail Snow Leopard installer was 10.6.3. It couldn't be used on Macs that shipped with later versions of 10.6.

There are even multiple special builds of the same OS X version that shipped with Macs with new hardware. The publicaly available version could not be used to boot those Macs even though the 10.x.y version numbers were the same.

So my situation with my model iMac is similar to that of the 2011 iMacs as those models that later shipped originally with OS X 10.7 Lion, they could still have OS X 10.6 Snow Leopard installed in the event someone didn't like OS X 10.7 because the hardware still supported installing an early OS X version.

MichelPM wrote:

So my situation with my model iMac is similar to that of the 2011 iMacs as those models that later shipped originally with OS X 10.7 Lion, they could still have OS X 10.6 Snow Leopard installed in the event someone didn't like OS X 10.7 because the hardware still supported installing an early OS X version.

Yes.