2014 15" MacBook Pro [4th,4C,H] + RTX 3070 @ 16Gbps-TB2 (ADT-Link R43SG * M.2-TB2) + Win10 [mbp2014ftw]
@joevt, Thats was i was thinking too, based on other posts in this forum, but apparently the M.2 Pinout includes plenty pins with 3,3V supply.
M.2 SSDs run off these 9x Pins of 3,3Volt of the M.2 adaptor. The original transcend SSD uses up to 3Ampere of 3,3V. So some kind of VRM must be in the transcend adaptor already, otherwise the SSDs would not be working. Also the thunderbolt receiver chip uses 3,3V. So 3,3V must be present in these TB2 M.2 adaptors already.
If i connect additional 3,3V i can probably burn the internal 3,3V supply of the adaptor or notebook, if i don't i will probably burn it due to taking too much current out of the displayport connector which is not meant to have serious power delivery.
The only reason for 3,3V supply not being present on the PCIe 4/16x Slot would be because the Pins are not connected. If this is the case it would be safe to connect an external Step down module to those pins (its easy to solder them from the back side).
If these pins are indeed connected, one possibility would be to shield them with kapton tape on the M.2 card and then connect the Step down module. If Kapton tape is too thick, i could still cut the traces on the adapter.
I will be driving the whole thing from a single +12V power source, so its no problem to solder an additional 3,3V step down module to that input. I don't think 5V power supply is even necessary, is it?
Probably i will start with something old like a gtx660/750ti for testing and later upgrade to a 1660ti or similar newer card if it works and the gpu marked madness has folded.
So from pictures in the Aliexpress comments i was able to make out how the 3,3V of the SATA+PCIe is connected. It seems this is connected all the way trough from the M.2 3,3V pins via the 3,3V Pins on the Sata to the 3,3V pins on the PCIe.
So what i will do once i get one of those transcend cases on ebay is to input a card without any additional 3,3v present, to see if it uses the 3v3 of the Thunderbolt2. And if that doesn't work with a low power draw pcie card, i will connect 3,3V over the sata connector and tape over the 3v3V pins on the M.2 Adaptor for safety.