Still scared of leaving Mountain Lion behind! Dual boot?

Question

I want to keep Mountain Lion because it works perfectly.  I just want to test Yosemite out of curiosity, and I am looking for the simplest and fastest way to do this. - Could I put the installer file in an SD card (8GB), boot from the card and install Yosemite from there?
- Could I install it from my Mountain Lion system and choose the SD card as the target drive during installation?
Any other idea?  I just don't want to mess up my current system.
Thanks in advance.
Alex

Answer 1

Create a small partition on your hard drive (but only if you don't use Windows.) Install Yosemite on this partition. You will need about 30-50 GBs.
To resize the drive do the following:
1. 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.
After the main menu appears select Disk Utility and click on the Continue button. Select the hard drive's main entry then click on the Partition tab in the DU main window.
2. You should see the graphical sizing window showing the existing partitions. A portion may appear as a blue rectangle representing the used space on a partition.
3. In the lower right corner of the sizing rectangle for each partition is a resizing gadget. Select it with the mouse and move the bottom of the rectangle upwards until you have reduced the existing partition enough to create the desired new volume's size. The space below the resized partition will appear gray. Click on the Apply button and wait until the process has completed.  (Note: You can only make a partition smaller in order to create new free space.)
4. Click on the Add [+] button below the sizing window to add a new partition in the gray space you freed up. Give the new volume a name, if you wish, then click on the Apply button. Wait until the process has completed.
You should now have a new volume on the drive.
It would be wise to have a backup of your current system as resizing is not necessarily free of risk for data loss.  Your drive must have sufficient contiguous free space for this process to work.
Download the Yosemite installer application from the App Store. After it downloads it will open automatically with an Install button. Click on the button. You will shortly see a screen in which a destination disk is presented. It will be your Mountain Lion volume, so DO NOT proceed yet. Click on the link under the disk icon labeled, "Show other disks." Now select the new partition/volume you created earlier. Click on the Install button. This will also work to install on an external drive. You won't have to do the partitioning thing from above. Just re-direct the destination to your external drive.