code.delx.au - refind/blob - gptsync/README.txt

   1 This directory contains the source code for gptsync, which is a program for
   2 creating hybrid MBRs (http://www.rodsbooks.com/gdisk/hybrid.html).
   3
   4 HYBRID MBRS ARE UGLY AND DANGEROUS HACKS, AND SHOULD NOT BE USED UNLESS
   5 ABSOLUTELY NECESSARY!
   6
   7 Despite their dangers, hybrid MBRs are useful because Windows interprets
   8 hybrid MBR disks as having an MBR partition table, whereas OS X and Linux
   9 interpret such disks as having a GUID partition table (GPT). Since Windows
  10 ties its boot mode to the firmware type (MBR/BIOS and GPT/EFI), a hybrid
  11 MBR enables Windows to boot in BIOS mode from a disk that's primarily a GPT
  12 disk, such as a Macintosh OS X disk.
  13
  14 Unfortunately, Apple uses hybrid MBRs as part of its workaround to enable
  15 Macs to boot Windows in BIOS mode while also supporting a standard EFI-mode
  16 boot of OS X. Many Linux distributions also install in BIOS mode on Macs,
  17 and so use hybrid MBRs; but it's usually possible to add an EFI-mode boot
  18 loader to get Macs to boot Linux in EFI mode, thus obviating the need for a
  19 hybrid MBR. Some Hackintosh installations rely on a hybrid MBR for reasons
  20 similar to those of OS X on a real Mac. Thus, you should use a hybrid MBR
  21 *ONLY* on a Mac that dual-boots with Windows or some other OS in BIOS mode
  22 or in very rare circumstances on other computers.
  23
  24 The version of gptsync provided with rEFInd is heavily modified from the
  25 original rEFIt version of the program. Most notably, it's "smarter" about
  26 creating a hybrid MBR: It prioritizes placement of Windows (FAT and NTFS)
  27 partitions in the MBR side, followed by Linux partitions. Other partitions,
  28 such as OS X's HFS+ partitions, might not appear at all in the hybrid MBR,
  29 whereas they generally do appear in hybrid MBRs created by rEFIt's version
  30 of gptsync. In the rEFIt version of gptsync, OS X partitions can crowd out
  31 FAT or NTFS partitions, particularly on computers with shared FAT or NTFS
  32 partitions, multiple Windows installations, or triple-boots with OS X,
  33 Windows, and Linux. The rEFInd version of gptsync also checks the
  34 firmware's author and warns if you're trying to run the program on anything
  35 but Apple firmware, since in most such cases creating a hybrid MBR is *NOT*
  36 desirable.
  37
  38 Although the Makefile supports building for both EFI (via the "gnuefi" and
  39 "tiano" targets) and Unix/Linux (via the "unix" target), the Unix build is
  40 currently broken; it returns a bogus error about an unknown GPT spec
  41 revision. If you want to create a hybrid MBR in an OS, you're better off
  42 using gdisk (http://www.rodsbooks.com/gdisk/), which provides much better
  43 control of the hybrid MBR creation process. gdisk may also be preferable if
  44 you have an unusual partition layout, many partitions, or specific
  45 requirements that you understand well.