Edit - 04.08.2018
A hint from @K.C. Lee together with my own findings helped making it work. The SD seemed to bleed voltage from either or all of the pins which were still high after turning off the VCC of the SD card.
Switching these all off after turning off SPI comm. worked. Quiescent power consumption is now 1.8uA.
Situation is as follows:
Putting the board into powerDown mode without the SD inserted does exactly that: Power down. Current consumption 1.6µA.
When using the SD, doing a few test writes, closing the file etc. and turning it off by pulling low the EN pin of the TPS27082 sleep current consumption is always 75µA.
When I pull the SD out it drops to the 1.6µA mentioned above. Following pic is the integrated high-side switch I control the SD power with:
|PIN (SD card)||VOLTAGE|