I tried to make an 8 GB partition on my hard drive so I could make it into a bootable drive, like Recovery HD but without the need to redownload the OS, but for some reason Disk Utility wouldn't let me make a partition smaller than around 40ish GB. After some trial and error in an effort to make a partition of no more than 8 GB, I wound up with quite a bit of unused free space.

I entered a bootable USB drive, prepared to wipe my HD into one volume and restore everything from a backup, but then I encountered this method for reclaiming free space that had me revert my HD back to its non-CoreStorage state. It worked perfectly to reclaim the free space and enable me to specify a disk partition size, but it seems too good to be true. (As in, I wasn't expecting it to be this easy so now I'm scared.)

My question: Is there an important reason why Yosemite always converts hard drives to CS volumes, such that undoing the conversion could come back to haunt me in the future? If so, how do I go back to CS?


There are no obvious reasons to convert parts of the main drive(s) to a CoreStorage scheme - except for Fusion drives and FileVault 2 encryption - until now.

To convert your classical to a CoreStorage scheme boot to Internet Recovery Mode, Recovery Mode or an external bootable (thumb) drive, open Utilities/Terminal in the menubar and enter:

diskutil list to get the Disk Identifier of the partition to convert (usually it's disk0s2).

Then enter:

diskutil cs convert DiskIdentifier with the DiskIdentifier found in the step above.

The conversion will take some seconds ( SSD) or up to minutes (HDD).

After the conversion is finished, quit Terminal, open Disk Utility and check the result for errors. Then restart to your main volume and continue enjoying your Mac.

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