Is there a possibility to downgrade from Yosemite

The only way to do this and retain your data is to roll in your pre-upgrade backup. Restore from Time Machine or whatever other backup from prior to the upgrade. That'll mean any changes since Yosemite was installed will be lost, so you'll want a backup of that — to give you some place to fetch those changes from.

Many of the folks requesting help about downgrading don't have a pre-upgrade backup, unfortunately.

Which means that there's no easy way to do this.

You'll end up wiping your disk and reinstalling OS X from distro, and re-loading your tools and files and applications, and you'll have trouble with data such as Photos, which use libraries and databases that tend to get updated with new OS X releases.

As for older releases on the Mac App Store, either look in the Previous Purchases in the Mac App Store and see if they're listed there, or contact Apple and acquire a redemption code.

Do a backup, preferably 2 separate ones on 2 separate drives.

Revert to a Previous OS X - Yosemite

OS X El Capitan: Revert to a previous OS X version

If you do revert, I'd use Setup Assistant to restore your data. This process takes a while, so do it when you won't need the computer for several hours, based on my experience.

If you never downloaded Mavericks, it is not available.

Contact Apple Support and have the serial number of your computer available. They will send you new original disks for a small fee.

Please download and install EtreCheck from http://etrecheck.com/

Run it and post the report here. Maybe after analyzing the EtreCheck report, we might be able to suggest an easier solution to get Yosemite working for you.

If the DVD's came with the computer, they are tied to that computer, and will not work with your computer. Contact Apple with your computer's serial number to get the correct DVDs.

HaveToDowngrade wrote:

As my wife bought a second new MacBook which now contains all of our data I'm not afraid of loosing what is on my old Mac - it's just some OpenOffice files. The problem is, that we deleted the backup to use the Seagate for the new laptop. We didn't think about this step before cause we didn't think about Yosemite not working on mine.

Sounds probably a little confusing. But if I do understand you right there will be no or at least no easy way to downgrade without CD or the backup. Right? Would a downgrade work with a CD if I'd find someone who still has his or hers?

If there's nothing on the disk that you want to preserve, and if you want to go to (for instance) Mavericks, then download the installer app and build a bootable USB, boot that, use Disk Utility to wipe the internal disk, and perform a standard install.

Details on building a bootable USB disk vary by release — newer releases have a tool for this. Here's the current sequence, using the installer and the createinstallmedia tool. Here's an older sequence. If your system is sufficiently senior to support it, here's the Snow Leopard DVD.

If the OS X version you're aiming for is not in your list of previous purchases, then you'll have to get a redemption code directly from Apple. Or pick a different release that you do have, of course.

The other approach is to resolve whatever happened with Yosemite. MacBook Pro 13" and 15" mid-2009 should work with El Capitan, assuming sufficient memory and disk storage is available.