As OS X 10.10 Yosemite DP1 runs awful in a virtual machine I wanted to install it onto my MacBook as a dual-boot configuration with OS X 10.9 Mavericks. My MacBook runs on a self-made FusionDrive configuration using a SSD and a regular hard disk. Here is a link to a HowTo for that FusionDrive stuff. As Mavericks is already installed, the hard part was to create partition for the Yosemite installation. What absolutely is not working is:

• Simple create two volumes on the FusionDrive using the “diskutil coreStorage createVolume” command
• Add a second partition using the disc-utility

The disc utility does create a partition, but it completely ignores the size you specified. For me it worked by booting from USB and using the “resizeStack” command (I got that hint from here).

diskutil cs resizeStack X Y 991G jhfs+ Media 120G

This command will resize the FusionDrive volume to 991G while creating a second partition with the size of 120GB. To exactly know how big your partitions should be, type:

diskutil cs list

It will show you the size of the current volume. If its, for example, 1000GB and you want to create a 20GB second partition, type:

diskutil cs resizeStack X Y 980G jhfs+ partitionname 20G

X should be the UID from the volume you want to shrink (also called Logical Volume (not Family)). Y is the UID from the physical volume you want to create the 2nd partition on. I used my regular disk for that to avoid reducing the performance of my FusionDrive. To make it clear: The second partition will not be part of the FusionDrive, so the performance may be slower that used to. To display the UIDs use the list command:

diskutil cs list

After that, install Yosemite on the newly created partition. After a successfull installation, you can choose which system to boot by holding the alt-key on boot. Worked for me and I hope it will work on your setup, too 😉