Pro Mini and SD card module strange behaviour

Hi all,

I built a distance logger using a Pro Mini 5V, ultrasound sensor, RTC, and Micro SD card module.

I've got everything working, however, I'm having trouble with the SD card module.

It's one of the cheap modules from Ebay: click here

The vendor claims it works on both 5v and 3.3v supplies.

I'm using a 5V Pro Mini so I connected SCK MOSI MISO CS GND to the right pins on the Pro mini, and the VCC from the module to the VCC of the Pro Mini (supplying 5v, checked with multimeter).

Now, everything works perfectly when the Pro Mini is powered via USB (usb-FTDI converter-Pro Mini).
It also works perfectly if I disconnect the USB, leave the FTDI module plugged in, and power from battery.
However if I power from battery without the FTDI module connected, it doesn't work. The problem seems to be communication with the SD module.

I had a good Google session, and while I can't pinpoint the problem I have a feeling it's related to logic levels.

Checking the SD modules I have, they do have a 3.3v regulator, and also what appears to be a level shifter (the SD modules I have are 100% identical to the picture in the link above).

I'm at a loss to be honest. Any help is appreciated at this stage.

Many thanks!

The vendor claims it works on both 5v and 3.3v supplies.

And backs that nonsense up by labeling the board 3.3V?

I don't understand your reply.
The -bad english- vendor page states:
"Compatible with 3.3V and 5V power input
board level conversion circuit, compatible with 3.3V and 5V signal input "

Are you saying the module isn't 3.3V compatible?

Reading some voltage when the module is working (writing to it) it reads:

gnd - vcc - miso - mosi - sck - cs
0 - 5 - 0 - 5 - 0 - 5

That's when the card is idle (ie not being written to).