I've been considering the Yosemite upgrade but especially after reading some of blue tooth lag problems, I'm wondering how to be able to go back to mountain lion.

Question

i have been considering upgrading to Yosemite from Mountain Lion but especially after reading about some of the bluetooth lag problems, i would want to be able to return to my current OS, just in case and am unsure what steps to take.

Answer 1

Drive Partition and Format
1. Open Disk Utility in your Utilities folder.
2. After DU loads select your hard drive (this is the entry with the mfgr.'s ID and size) from the left side list. Click on the Partition tab in the DU main window.
3. Under the Volume Scheme heading set the number of partitions from the drop down menu to one. Click on the Options button, set the partition scheme to GUID then click on the OK button. Set the format type to Mac OS Extended (Journaled.) Click on the Apply button and wait until the process has completed.
4. Select the volume you just created (this is the sub-entry under the drive entry) from the left side list. Click on the Erase tab in the DU main window.
5. Set the format type to Mac OS Extended (Journaled.) Click on the Security button, check the button for Zero Data and click on OK to return to the Erase window.
6. Click on the Erase button. The format process can take up to several hours depending upon the drive size.
To clone your existing system:
Clone Yosemite, Mavericks, Lion/Mountain Lion using Restore Option of Disk Utility
Boot to the Recovery HD:
Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.
     1. Select Disk Utility from the main menu then press the Continue
         button.
     2. Select the destination volume from the left side list.
     3. Click on the Restore tab in the DU main window.
     4. Select the destination volume from the left side list and drag it
         to the Destination entry field.
     5. Select the source volume from the left side list and drag it to
         the Source entry field.
     6. Double-check you got it right, then click on the Restore button.
Destination means the external backup drive. Source means the internal startup drive.