I have just completed a project to upgrade the innards of my mid-2010 iMac with a new HDD and a new SSD. The OS is now installed on the SSD, but I want to make this a fusion drive with the HDD. When I try to follow the Terminal instructions to create one drive, it cannot unmount the SSD (presumably as this is running OS X). Can I simply add the HDD to the same volume on the SSD, or do I have to go through the pain of booting from an external drive? Thanks!