There are 2 scenarios here.
1 - a recovery partition was created but is not available in the startup manager because core storage logical volumes were also created by the yosemite installer which is preventing the recovery partition from showing up in the startup manager. In this case the workaround which usually works (unless you have a fusion drive where much more work will be required):
If the yosemite installation creates a core storage logical volume you can revert it to get partitions back to normal by running these 2 commands in terminal.
diskutil cs list
and then
diskutil coreStorage revert lvUUID
where lvUUID is the last lvUUID reported by the previous Terminal command.
2 - no recovery partition was created. In this case you can create one yourself.
To make the recovery partition:
Download the Lion Recovery Update from
http://support.apple.com/kb/dl1464. Make sure it is in your downloads folder. If you still happen to have the yosemite installer app somewhere, right click on it and click Show Package Contents. Go to Contents/SharedSupport/. Copy the InstallESD.dmg file into your Downloads folder. If you don't still have the installer, you can get it again by redownloading from your purchases tab in your mac app store.
Download and decompress the file recovery.sh.zip from
http://4unitmaths.com/recovery.sh.zip and move recovery.sh into your Downloads folder if it's not there already.
Open Terminal and type the following commands:
chmod +x ~/Downloads/recovery.sh
sudo ~/Downloads/recovery.sh
Wait a few minutes for it to finish and return back to a prompt. Reboot with holding down the option key to test your 10.10 recovery partition.
