MBR not detected in some cases when booting from SD to SATA adapter
 

I'm working on building an embedded system where large amounts of storage are unnecessary and cost prohibitive, and reliability is more important than size or performance. At most, I need a few hundred megs per machine, and the cheaper and more reliable, the better.

So I found these SD/MMC to SATA adapters that seemed to fit the bill perfectly. I tried writing a bootable disk image with GRUB 2 directly to the SD card with dd, and after inserting it in the adapter, the machine only says "Reboot and select proper boot device" upon trying to boot. The adapter shows up both in the BIOS setup, and the boot menu.

The image boots fine in a VM, or when written to an HDD. The adapter is also not bootable in other machines, so I know it's not isolated to the one machine.

The strange thing is that when I wrote an archboot image (a hybrid image using isolinux) to the SD card, it booted fine. I can't for the life of me imagine what would cause the MBR from one bootloader to be completely invisible to the BIOS, and another to work just fine.

I also tried installing Arch to the SD card with syslinux instead of GRUB 2, and I encountered the same issue with the MBR not being detected.

I've got to believe this is some sort of hardware quirk. Any idea what could be causing this?

EDIT: I should add that the machine was set to boot in legacy mode, not UEFI, and I tried both AHCI and IDE SATA modes.