Understanding MBR, APM, & GPT

MBR, GPT, and APM. (These are the ones Apple's Disk Utility can create.)

 

MBR (Master Boot Record) is used historically by Windows (and, therefore, by most computer manufacturers). I'm not certain if you can boot a Mac from an MBR disk; I don't think so.

GPT (GUID Partition Table) is generally used in conjunction with Intel's EFI BIOS replacement. As such, it is mostly used by Apple currently in their Intel Macs. Any Mac running 10.4 or above can read a GPT disk; only Intel Macs can boot from them. If a computer has EFI, you can boot Windows on a GPT disk.

APM (Apple Partition Map) is what Apple used before the Intel switch. It's been used since 1989 in the Macintosh SE. Intel Macs can boot from and read APM disks.

I'd say the assertion you made in the question ("GPT is more useful than MBR") is incorrect: you can certainly boot more computers with an MBR disk than one using GPT. GPT is the way of the future, but it may take awhile for the major PC manufacturers to switch to EFI from BIOS.

Your Time Machine disk is almost certainly uses GPT, but that's totally irrelevant as you'll never boot from it.

• Intel-based Macs all use EFI, and can only boot from GPT drives.
• Booting from MBR is supported by all non-Mac x86 PCs. Booting from a GPT disk requires UEFI firmware.
  (This requirement doesn't apply for using MBR/GPT on data disks; in that case, only OS support is necessary.)
• The MBR is 512 bytes in size; GPT can grow depending on partition count.
• MBR uses the obsolete cylinder-head-sector addressing format, apparently.
• There is only one MBR per disk. GPT keeps a backup copy.
• MBR limits the whole disk to 2 TiB; in GPT the disk can be up to 16 EiB or 8 ZiB.
• MBR is limited to four primary partitions. GPT can have 128 partitions.
• To get around the 4-partition limit, the fourth partition in MBR is often an "extended partition", pointing to a linked list of "logical partition" records scattered between actual data, introducing even more weak points. GPT doesn't need such workarounds.
• The boot loader is part of the MBR. (Sort of. Almost all bootloaders are too large to fit, so the part in MBR just loads a stage-2 bootloader from a partition.) There can only be one bootloader in the MBR, resulting in conflicts when installing dual-boot systems. Also, the MBR bootloader code seems to be specific to the x86 architecture.
  On the other hand, GPT uses a dedicated FAT32 partition for bootloaders (potentially multiple) and other EFI tools. The partition contents can be easily managed from any OS.
• MBR partitions have a one byte long "type" code, which is too small to be useful, so the type often has to be guessed by the OS. Windows NT introduced a four-byte unique ID, but this is non-standard.
  In GPT, each partition has a type GUID, an unique GUID for identifying the partition itself, and a textual name.
  

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