How to rename (main) hard disk back to default? And is the disk default really LVG?
The naming scheme for non-CoreStorage devices/volumes looks like this:


(hard-coded) name of the disk device


EFI (invisible in non-tweaked Disk Utility)
Volume 1 (usually Macintosh HD)
Volume 2 (optional)
... (optional further volumes)
Recovery HD (invisible)



And the naming scheme for CoreStorage devices/volumes looks like this (a Fusion-disk has an additional invisible EFI partition, an invisible Boot OS X volume and a second invisible Physical Volume):


name of the Logical Volume Group


EFI (invisible)
name of Logical Volume 1 (usually Macintosh HD)
name of Logical Volume 2 (optional)
... (optional further volumes)
name of the Physical Volume (invisible)
Recovery HD (invisible)



The names of the Logical Volume Group and the Logical Volumes usually derive from former volume names or is just "Untitled" for Physical Volume(s) but the hard-coded physical device name is rarely used (I have seen it only once, probably the owner renamed it).

Renaming the Logical Volume Group:

To rename the Logical Volume Group use:

diskutil cs rename lvgUUID|lvgName newName


To get the lvgUUID just enter diskutil cs list. It's the first UUID listed. Alternatively you may use the Logical Volume Name. I don't know whether this works if you have several equally named LVGs or PVs. Therefore I always prefer the UUID.

To get the previous visible disk device name open "About your Mac" -> System Report -> Hardware -> SATA/SATA Express and search for your device.

Reverting CoreStorage to non-CoreStorage:

To revert the CoreStorage non-destructively to a traditional non-CoreStorage volume boot to Recovery Mode/Internet Recovery Mode or an OS X thumb drive.

Open Termínal.app and enter:

diskutil cs list


to get the Logical Volume UUID (usually it's the last listed) and enter:

diskutil cs revert lvUUID


to convert your CoreStorage device to non-CoreStorage.

Though the command converts CS to non-CS non-destructively it's always a good advice to have a current backup of your main volume.

You mustn't do this with Fusion drives or you will destroy all your data.