256GB M.2-drive, SM2258XT and Sandisk 453E99B37A6B00. Controller has probably encrypted the NAND so I do not want to desolder it as first step, but still I'd like to take a look at the contents.

Since there is one free footprint (only one NAND on PCB), is there a possibility to solder some points on a monolith-adapter and access the NAND? According to this link it is BGA304/BGA272. I do not have an adapter for that and I do not find the pinout in ONFI-specifications. In VNR it says "Custom pinout" for this chip.

So how do I find the pinout so I can hook it up to a monolith adapter and try to read the contents?

It use BGA-272 pinout. You will find it at ONFI document.

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Flash Killer - everyday new resources (pinout, XOR, ECC,config) for flash devices

ONFI BGA-272 does not match the pinout. BGA-316 is close, but still no exact match.

Or maybe I'm reading the document wrong. Gray areas = N/C?

Your chip in fact it is BGA-304. But from my notes it have compatible pinout with BGA-272. But of course verify me, I can be wrong.

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Flash Killer - everyday new resources (pinout, XOR, ECC,config) for flash devices

@arvika: I sent you a mail because I decided this one is a bit out of our league.