Hi there!
We have some good news, a guy from the 4pda forums figured out how to make the missing SD slot work. I've replicated that on my M9s box a few days ago and it works like a charm. You can see original post in Russian here.
So, basically, this TV box uses the same reference design for that circuitry as Armsom Sige5 and other boards. You can check the diagrams for Sige5 to get an idea. You need two capacitors (10uF and 100nF, on the power line) on the SD slot side (two smaller pads) and a LPW5202ADB5F chip and some resistors and caps on the other side.
LPW5202ADB5F is a power switch chip with current limiter, can be bought on Aliexpress for like $5-6 for 20 pcs, also need these components:
1x 10k resistor - controls current limiter (this value is for 0.7A according to data sheet, lower values for higher current, or 3.6k = 2A),
2x 10uF caps on power input/output lines
1x 100k - pull down resistor on the 'enable' contact
1x 2k - limiting resistor from the CPU to the 'enable' line
6x 22 Ohm resistors for the data lines between CPU and SD card slot
In theory, it's possible to put a zero-ohm resistor between in and out instead of that switch chip, in that case SD slot will be powered all the time and won't have a current limiter.