This is on the adafruit website:
There are a few things to watch for when interacting with SD cards:
One is that they are strictly 3.3V devices and the power draw when writing to the card can be fairly high, up to 100mA (or more!)
Guess you need to adjust for the different voltages but the answer will still be over 40 mA. I just took the short-hand = 'bad idea'
Switched the ground rather than Vcc because that is what others seemed to do! [I am really a cattle veterinarian wanting to monitor the environment in cow sheds]; also the way my boards have evolved that was a bit easier than Vcc.
I am on the road for the next day or so but you have given me some more ideas for tinkering to try when I get back.
Many thanks for your help - I will let you know how it goes.