[Buildroot] Grub2 on x86_64 build works in VM, but not on target

[Jimmy at 86Pixels]

Good morning all,

I've got a question here that has had me stumped for a bit.  I'm
targeting the pc_x86_64 platform, and the build and everything finishes
perfectly, and I'm able to run my disk.img in QEMU and get the grub
screen, and subsequently boot into the OS using the -hda argument.  I
then flash disk.img (auto generated by buildroot) to a SATA SSD and the
kernel eventually panics because it can't mount the rootfs. Everything
is at default settings, so root is set to /dev/sda1.  Grub2 detects the
drive as (hd0,msdos1), and I'm able to read the contents of the
drive/partition from grub.

The error is Unable to mount root fs on unknown block (8,1): error -6. 
The boot screen DOES seem to be detecting sda, and again, I can read its
contents from the grub command line.

I realize this is a question straddling grub vs. buildroot support, but
I didn't know if any of you had seen anything similar using buildroots
default configurations for this particular target, or if this was a
known issue.

Thank you so much in advance! Any pointers are appreciated.

Jimmy