Apple introduced a new volume manager in OS X 10.7 named Core Storage, which is a layer between the disk partition(s) and the file system. It was developed to make Apple’s Fusion Drive technology work, which allows several partition on multiple drives to be used a single logical volume.

OS X 10.6 and older can’t see Core Storage volumes, which is something all Mac support engineers should know, especially when booting from external drives with multiple OSes. OS X 10.7-10.10 and later can see both Core Storage and non-Core Storage volumes.

With OS X 10.7 and later, if you turn on FileVault, it will be convert the boot volume to Core Storage. Similarly, if you set a volume to be encrypted in the Finder or Disk Utility, it will be converted to Core Storage.

Macs that ship from Apple with OS X 10.10 pre-installed have a Core Storage boot volume even though FileVault is not turned on by default. Many people have found that after upgrading to Yosemite their boot volumes are converted to Core Storage even though FileVault was turned off.

To view a list of all volumes using Core Storage, run this command in the Terminal app (requires OS X 10.7 or later):
diskutil coreStorage list

So if you are in a situation where a Mac running OS X 10.6 can’t see a volume, it’s possible the volume could be using Core Storage.