[Allyoopski]

I recently upgraded to Yosemite, and only just now looked at the Disk Utility, and noticed the format of my drive reads as “Logical Volume Group” instead of Mac OS Extended that it was before the upgrade. I did not do anyhting to the drive. Basically what I want to know is, does this matter? It is a 1TB HDD in a macbook pro.

[P]

This is normal, and you don't have to bother about it.

Basically, Apple has developed a so-called logical volume manager called Core Storage. An LVM is a piece of software that sits "between" the disk and the filesystem, in this case Mac OS Extended. It is normally used to group multiple disks together and tasks like that, but Apple is also using it for a lot of the advanced disk features recently added to Mac OS X, such as Fusion Drive and Filevault 2. Previously this meant that you had to convert your disk to use Core Storage when you turned on something like Filevault 2, which took some time. To save that time, Apple now converts your disk to a Core Storage group during installation of Yosemite, and that is what you are seeing in Disk Utility.

Core Storage apparently adds a few features to make the file system a little more reliable, but I haven't seen anything more about how well that works.

[Atheist]

What's lame though is that Disk Utility can't operate on Logical Volume Groups. If you want to take an existing Yosemite installation and start from scratch the only way you can erase and repartition the drive is to use the "diskutil cs" commands from Terminal. For me it was no big deal since I've been using Unix since the early 80's but for the average Joe it will be daunting.

If anyone is interested, use "diskutil cs list" to get a list of the logical volume. You'll see something like this:

Code:
CoreStorage logical volume groups (1 found)
|
+-- Logical Volume Group 6E63414B-D477-4503-B958-E7CBC478AA0D
    =========================================================
    Name:         Yosemite
    Status:       Online
    Size:         2999733223424 B (3.0 TB)
    Free Space:   35680256 B (35.7 MB)
    |
    +-< Physical Volume 0B47C099-7A46-4C60-BF04-1C5535269DD2
    |   ----------------------------------------------------
    |   Index:    0
    |   Disk:     disk5s2
    |   Status:   Online
    |   Size:     2999733223424 B (3.0 TB)
    |
    +-> Logical Volume Family 0AA21496-D33B-4997-8F97-3456BCF1907A
        ----------------------------------------------------------
        Encryption Status:       Unlocked
        Encryption Type:         None
        Conversion Status:       NoConversion
        Conversion Direction:    -none-
        Has Encrypted Extents:   No
        Fully Secure:            No
        Passphrase Required:     No
        |
        +-> Logical Volume F7BD87D4-1ADF-4AC0-BEA5-3566E5A12191
            ---------------------------------------------------
            Disk:                  disk6
            Status:                Online
            Size (Total):          2999361994752 B (3.0 TB)
            Conversion Progress:   -none-
            Revertible:            Yes (no decryption required)
            LV Name:               Yosemite
            Volume Name:           Yosemite
            Content Hint:          Apple_HFS

Copy the Logical Volume Group UUID and then delete the LVG with this command: "diskutil cs deleteLVG 6E63414B-D477-4503-B958-E7CBC478AA0D"

Now Disk Utility will have full access to the drive.

[osiris]

Just an observation, but my disk util still says Macintosh OS Extended (Journaled) under Yosemite.

[Atheist]

Even though it's a Logical Volume Group it will still say Mac OS Extended (Journaled). When you look at the disk in Disk Utility it will indicate it's a LVG and the format (on the bottom of the window) will show "Logical Partition".

[P]

There is also the fact that Macs which do not support the AES instructions are apparently not converted automatically. This includes a good chunk of the still supported Macs: iMacs, minis and MBAs got that support with the 2011 models, and the MBPs in 2010. Some MPs got that in 2010 as well, but a couple of the low-end models lacked it until the launch of the trashcan MPs in 2013.

[bradnic]

great post Yosemite Auto-Converts to Core Storage - HCS Online - HCS: Problem-solving by Apple Certified Professionals