People wanting to boot Linux from their micro SD on their Surface Pro (basically the same as me wanting to boot Windows 8.1 from uSD slot on my Win10 tablet; will confirm later if it works or not when my uSD card arrives):
(BTW, anyone following this thread, a word of caution: Don't do anything to overwrite your tablet's Win10 bootloader. How I will get Win 8.1 onto my tablet is to install it on my laptop by native booting via a vhd, sysprep that vhd image, transfer the vhd file to a uSD card, add a boot option to it on the tablet by using bcdedit to edit Win10's bootloader...)
Pretty sure the uSD slot is attached via USB, so it's basically just "boot from USB external storage".
Another way you could have done it is to install the Linux bootloader (GRUB stage 1) to the internal storage, and then have that chainload to the uSD card. That should work even if for some reason the firmware doesn't want to boot off uSD normally.
(BTW, anyone following this thread, a word of caution: Don't do anything to overwrite your tablet's Win10 bootloader. How I will get Win 8.1 onto my tablet is to install it on my laptop by native booting via a vhd, sysprep that vhd image, transfer the vhd file to a uSD card, add a boot option to it on the tablet by using bcdedit to edit Win10's bootloader...)
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