Oh yes - on u-boot did you try my version that enabled scanning of both the eMMC and the sd card? Would be useful to know how far it goes. Alas I suspect mainlining all of armian patches looks a challenge to me. Instead focus of the minimum changes needed so that u-boot can boot from either the eMMC or the sd card.
On the sd card errors when there is no sd card installed. Yes this should be handled with the CD = Card Detection. Now its not clear if this is enabled on the TB S - the schematic
http://dlcdnet.asus.com/pub/ASUS/mb/Linux/Tinker_Board_2GB/Tinker_Board_Schematics.zip doesn't seem to show the connection to the sd card! Pity as then we could see what is wired. There is a standard way to describe CD given here
https://github.com/torvalds/linux/blob/master/Documentation/devicetree/bindings/mmc/mmc.txt - we could force it to do polling, but that may be what is doing right now. On the other hand it may be the driver adjusting the phasing:
https://github.com/torvalds/linux/blob/master/Documentation/devicetree/bindings/mmc/rockchip-dw-mshc.txt - which would explain why it quietens down after a bit. Le me know if you want me to try out the device tree.
On the device tree, the maining kernel and debian are slightly different. The mainline kernel just has:
$this->bbcode_second_pass_code('', '&sdmmc {
bus-width = <4>;
cap-mmc-highspeed;
cap-sd-highspeed;
card-detect-delay = <200>;
disable-wp; /* wp not hooked up */
pinctrl-names = "default";
pinctrl-0 = <&sdmmc_clk &sdmmc_cmd &sdmmc_cd &sdmmc_bus4>;
status = "okay";
vmmc-supply = <&vcc33_sd>;
vqmmc-supply = <&vccio_sd>;
};')
Whereas Debian instead has
$this->bbcode_second_pass_code('', '&sdmmc {
bus-width = <4>;
cap-mmc-highspeed;
cap-sd-highspeed;
sd-uhs-sdr12;
sd-uhs-sdr25;
sd-uhs-sdr50;
sd-uhs-sdr104;
card-detect-delay = <200>;
disable-wp; /* wp not hooked up */
num-slots = <1>;
pinctrl-names = "default";
pinctrl-0 = <&sdmmc_clk &sdmmc_cmd &sdmmc_cd &sdmmc_bus4>;
status = "okay";
supports-sd;
vmmc-supply = <&vcc33_sd>;
vqmmc-supply = <&vccio_sd>;
sd-uhs-sdr104;
};')
So its just a lot more verbose about the modes that the sd card slot can handle. Anyway aside from disabling write protection, neither does anything unusual with card detect ...