If two drives are marked bootable what happens?

[Chris Green]

A friend has been having trouble with a SATA SSD that his system won't
recognise so I have been playing with it a bit to see if I can work
out what the problem is.

This question isn't really related to the above problem.  I plugged
the SATA SSD into a system of mine which happens to have an eSATA
connector and 'fdisk -l' then shows:-

    Disk /dev/sda: 465.8 GiB, 500107862016 bytes, 976773168 sectors
    Units: sectors of 1 * 512 = 512 bytes
    Sector size (logical/physical): 512 bytes / 512 bytes
    I/O size (minimum/optimal): 512 bytes / 512 bytes
    Disklabel type: dos
    Disk identifier: 0xff18eec4

    Device     Boot     Start       End   Sectors   Size Id Type
    /dev/sda1  *           63 964189169 964189107 459.8G 83 Linux
    /dev/sda2       964189170 976768064  12578895     6G  5 Extended
    /dev/sda5       964189233 976768064  12578832     6G 82 Linux swap / Solaris


    Disk /dev/sdb: 149.1 GiB, 160041885696 bytes, 312581808 sectors
    Units: sectors of 1 * 512 = 512 bytes
    Sector size (logical/physical): 512 bytes / 512 bytes
    I/O size (minimum/optimal): 512 bytes / 512 bytes
    Disklabel type: dos
    Disk identifier: 0xfa947ad3

    Device     Boot Start       End   Sectors   Size Id Type
    /dev/sdb1  *     2048 312580095 312578048 149.1G 83 Linux


The system has booted from /dev/sda1 (its internal disk drive), why does it 
choose to boot from this drive rather than /dev/sda1 since both are marked
bootable?

-- 
Chris Green