Micro SD card to log reading from four force sensors connected to the microcontroller. Controller logs reading to the SD card every 15 minutes and goes to sleep.
You seem to be closing the FAT when you finish writing, so the SDCard should automatically go into IDLE mode (clocks stopped). Make sure you do a read at the end of your write to ensure you cleared any write/erase. I'm not sure what SDCard you are using but here is a typical spec. The IDLE current is about 150 uA.
If you want to get lower than this then you could use a digital pin plus FET switch to turn the Vcc off when you are asleep, and turn it back on before initializing the FAT when you wake up.