root partition is flagged as bootable, but it isn't booting

Question

I have two physical drives. I put a root partition on one of those drives during installation. I even told it to use that root partition for booting. When I look at my partition with gparted, it even says the partition is marked with the "boot" flag.

However, unless I manually go to the boot option screen at start-up and select the drive (when the hardware logo appears), the computer will not boot. It will just give me a blinking underscore.

Here is the output from 'fdisk -l'

Disk /dev/sda: 750.2 GB, 750156374016 bytes
255 heads, 63 sectors/track, 91201 cylinders, total 1465149168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk identifier: 0x00067e03

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1            2048    31250431    15624192   82  Linux swap / Solaris
/dev/sda2        31252478  1465147391   716947457    5  Extended
Partition 2 does not start on physical sector boundary.
/dev/sda5        31252480  1465147391   716947456   83  Linux

Disk /dev/sdb: 60.0 GB, 60022480896 bytes
255 heads, 63 sectors/track, 7297 cylinders, total 117231408 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
Disk identifier: 0x000c98cc

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1   *        2048    39063551    19530752   83  Linux