Okay, by digging through the codes, I have pin pointed the failed initialization (after one SD power down and sleep) to this line in SDFile.openRoot :
https://github.com/arduino/Arduino/blob/master/libraries/SD/utility/SdFile.cpp#L550
This can probably be fixed by implementing some new method that resets the state of the SDClass object that can be called in init, or before doing a new init, or before (or after) turning the SD card off and before sleeping.
What do you think would be the best way of action ?
Cheers,
Robin