Apple_Boot partition created on external drive

disk-utilityexternal-diskpartitionrecovery

I used Disk Utility to partition my external drive into four partitions: two in Apple file system (called H1 and H2) and two in FAT (called F1 and F2). However, I found by diskutil list that an Apple_Boot partition called "Boot OS X" is automatically created:

0:      GUID_partition_scheme
1:                        EFI EFI
2:          Apple_CoreStorage H1
3:                 Apple_Boot Boot OS X
4:                  Apple_HFS H2
5:       Microsoft Basic Data F1
6:       Microsoft Basic Data F2

Why is there an Apple_Boot partition? Does it mean that now I can boot a Mac using this external drive? But the size of this partition is much smaller than my recovery partition "Recovery HD" on the internal drive, so I don't think this is a full copy of the recovery partition.

I want a "clean" partition without introducing Apple_Boot or EFI. (I remember from the old days there was a limit on the number of primary partitions, so having six partitions makes me feel uncomfortable.) To do this, can I use Windows to partition my external disk, and then use my Mac only to format H1 and H2? Can I still use H1 for Time Machine if I do so?

Best Answer

You created one HFS partition, and one Core Storage partition, on which an HFS partition resides. The difference is that a Core Storage partition can be encrypted. I've never found a use for the Apple_Boot partition, although it is replaced with the recovery partition when you install OS X. You needn't worry about running out of partitions. The limit is ridiculously high these days. The EFI partition is part of the GUID Partition Table format by default, and should be kept. Neither Apple_Boot nor EFI take up much space, so I'd shy away from deleting them without a good reason.

Related Solutions

Partition HD for triple boot (or move Apple_Boot Recovery HD from a primary partition)

You're getting confused between the MBR and GPT partitioning schemes. Intel Macs generally use GPT, but for Boot Camp, they also add a so-called Hybrid MBR for booting Windows, which duplicates all or some of the partition entries. GPT can have a large number of partitions (typically, enough space is reserved for 128 partitions), but as MBR only supports 4, if you have more than that, you can't represent all of them in the MBR. Note that one partition of the EFI type 0xEE is required - this normally covers the whole disk, but for hybrid MBR, it usually only covers the GPT itself and the 200MB EFI partition at the start of the disk. So that gives you 3 usable MBR partitions, which Windows will see.

You don't mention what your third OS should be for triple-boot; if it's another installation of OS X, you have nothing to worry about. Just install it as normal, it will use GPT entries 5+. If you want another BIOS-based OS, such as Windows or most Linux distros, you'll probably need to mess with the hybrid MBR yourself. I'd do all the partitioning in OSX's Disk Utility, as it will update both the GPT and MBR tables simultaneously. Just make the new Windows or Linux partition FAT32 to begin with. Then, if your MBR covers the Recovery HD but has no entry for your new partition, get the exact positions and sizes of the GPT partitions using

sudo gpt -r disk0

This should list all 5 partitions.

In fdisk (or so), delete the MBR version of the Recovery HD, and create a new partition with the exact position and size of your new (5th) partition. Windows should now see this partition instead of the Recovery HD. You can change its type, format it, etc. as usual. Just don't try to resize it from fdisk or Windows Generally, make sure that all partitions that exist in the MBR except the 0xEE EFI partition map exactly onto a GPT partition.

This is all a bit dangerous so make sure you have backups of everything.

MacOS – Can’t modify partitions

Thanks @Tetsujin. The listing looked a little screwy so I was a little hesitant to go mucking around with the command-line diskutil. But I ended up doing a

diskutil corestorage list

and then

diskutil corestorage delete [UUID of 50GB Logical Volume Group]

GUI Disk Utility correctly saw the 50 GB of empty space as part of the other Logical Volume Group and I was able to delete it and merge it back in fine.

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