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I have learnt that the BIOS collects the most diverse hardware data and create a complete list of all of the device’s available drives immediately after being powered on.

When this process is complete, the firmware goes through the data carriers found in sequence, checking for a bootloader by means of a special signature – the so-called boot signature (or ‘boot record’). The search always starts on the removable media (CD/DVD, USB stick, external hard drive, etc.), followed by the hard-coded drives. With the latter, the bootloader and its signature is generally in the master boot record (MBR).

My question - Since the BIOS checks the boot signature value in all the detected storage devices one by one, does that mean every (storage media detected by the BIOS here) must have a Master Boot Record irrespective of whether the device is bootable?

Giacomo1968
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manoj kumar
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2 Answers2

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Well, no, they don't need to have a MBR if they aren't meant to be bootable.

Most operating systems after MS-DOS do not rely on the BIOS of disk access. As soon as the OS has started booting, it'll use its own drivers for talking directly to the storage devices – and will be able to use them regardless of whether the BIOS had detected them in any way or not.

That said, most will have a MBR or something resembling it.

  1. The MBR doesn't only contain BIOS boot code; it also contains a partition table. Any disk using the traditional MBR partitioning will have the boot signature, even if it doesn't have any boot code there.

  2. UEFI uses a different partition table format (the GPT), but specifies that every GPT-partitioned disk should still have a "protective" MBR (with empty boot-code area and a special partition), to avoid outdated tools incorrectly thinking that the disk is empty.

    (To be clear, GPT disks can be used on BIOS systems, as it's not the BIOS but the OS that handles the partitioning; and the protective MBR can even hold boot code, so it's possible to make such disks bootable on BIOS systems as well.)

Finally, practically all new PCs for the last couple of years have used UEFI firmware – not BIOS; the native UEFI boot process is completely different and doesn't use MBRs. (Though for a long time most UEFI PCs could still recognize and boot from disks in the BIOS way, but that's now slowly disappearing from new models.)

grawity
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Your posting is confusing because it contains two different questions.

As for the question in the title:

Does every storage device have MBR?

Yes. The old MBR-based partitioning scheme has been replaced by the modern GPT scheme which includes a protective MBR to prevent old operating systems to perform write operations on the drive.

There is one rare exemption called "superfloppy" which does not play a role in the boot process of desktop computers.

As for your second question:

My question - Since the BIOS checks the boot signature value in all the detected storage devices one by one, does that mean every (storage media detected by the BIOS here) must have a Master Boot Record irrespective of whether the device is bootable?

Technically no. What you described in your posting is the old boot process of an old BIOS. A modern bios uses a different boot process according to "UEFI". It does not have to care about MBR structures on disk because the GPT contains all necessary information. A modern UEFI-Bios only has to find the bootloader on the EFI system partition.

r2d3
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