I'm sorry, I can't identify the mystery part.
"91512" is probably a dual step-down converter. It appears to have a pinout similar to Rohm's BD9152MUV, but the latter requires an input voltage range of 4.5V - 5.5V. Your IC would be powered from 3.3V. The "5211B" marking suggests a YWW date code of 2015, week 21. I'm betting that this IC produces the Vcore and Vio supplies for the flash controller. The Vio supply might also be used by the NAND array.
The date code on the PCB appears to be 1523 (below the sticker) and the date on the sticker is 16 JUL 2015.
The markings on the IC above "MDK" appear to be ...
This would suggest a YWW code of 2015, week 17. The part code is probably "841".
Your mystery part has a "531" in its markings, but I can't see how this could be a YWW code since it would be more recent than the manufacture date on the sticker. The fact that there are two of these parts precludes it as a temperature sensor. I can't see it being a power supply supervisor IC for the same reason. The heavy trace appears to be the supply pin, but only two of the remaining pins appear to be connected. The ICs don't look like LDO regulators, either. I can't make out the ground pin, assuming there is a ground connection.
Toshiba Solid State Drive HG6 Series datasheet (THNSNJ256GVNU):
http://www.farnell.com/datasheets/1852757.pdfBD9152MUV, Rohm, 1.5A dual step-down switching regulator:
http://rohmfs.rohm.com/en/products/databook/datasheet/ic/power/switching_regulator/bd9152muv-e.pdf