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The computer I am using right now has 6 physical disks attached. Using the BIOS's "select boot device" functionality I have discovered that I can boot off 3 of these disks into the exact same bootloader. This means that all of these disks have the bootable flag set on their MBR, even though I had been under the impression that only 2 of them are in fact bootable.

My question is: is there an easy way to check which disks have a bootable MBR with a command line tool or some OS utility? I am aware of the DISKPART tool and its basic functionality, but I haven't found how to view the information I need.

The OS is Windows 7 x64 Ultimate with SP1.

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2 Answers 2

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Open the Start Menu and write "compmgmt.msc". This should find one hit ("Computer Managment"; or similar, not using the English Version).

In there, you'll find one sub section dedicated to storage devices/disks.

There you'll find a list of all existing and active partitions together with their flags listed under "Status":

Screenshot


To determine whether a disk has a MBR or a GPT open diskpart and run list disk.

To show more Information, select a disk using select disk 0 (or any other number shown in the list), then type detail disk.

In a similar way partition Information can be shown once a disk has been selected using list partition and Detail partition.

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MBR bootability is an attribute of the disk, not of a partition. Therefore I highly doubt a list of partitions can provide this information. Plus, you haven't really given an answer. –  Jon Nov 25 '12 at 22:47
    
My bad, thought you're talking about having a partition set as "active" - adding some lines for diskpart... –  Mario Nov 25 '12 at 22:53
    
As far as I can tell detail disk doesn't do that either. It just tells you if that disk is the one you booted from. –  Jon Nov 25 '12 at 23:12
1  
Reread a bit on the topic. There's no additional flag as far as I'm aware (at least nothing standardized), and it should be up to the partition table (see first part of my answer). But I'm actually curious, whether there's something I missed. –  Mario Nov 25 '12 at 23:19
    
I 've been reading Wikipedia on MBR, and it now looks like you are correct and I was under the wrong impression. But I 'd certainly like to get a more authoritative answer. –  Jon Nov 25 '12 at 23:28

In the MBR style, the so-called 'bootable flag' resides in the first byte of the partition entry. If the first bit is set, the partition is marked bootable. For compatibility reasons, the other seven bits are usually left at zero, yielding 0x00 for non-bootable partitions and 0x80 for bootable ones.

Example MBR

Above is an example of part of a master boot record. Notice the signature 55 AA at the end. Highlighted in blue is the partition table. Each of the four entries is sixteen bytes long. All but the first are non-bootable (and empty).

Although Windows does not, to my knowledge, have a built-in tool for displaying raw disk data, it is possible to extract this information through DiskPart. Windows's non-standard naming obscures it somewhat, but both here and in the graphical disk manager, the flag is called 'Active'.

DiskPart

Shown above are details of an apparently bootable partition. The flag can also be set with a subsequent active or inactive command, or through the graphical disk manager by right-clicking the partitions, then choosing 'Mark partition as (in)active...'.

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