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@MK Omega2 uses NOR flash, there's a discussion here:
https://community.onion.io/topic/4987/enabling-reset-hack-on-boot?_=1714985949781
You can also check some of my posts for an image of the Omega without the top cover so you can see the actual chip.
Not sure about the specifics of the difference between du and df however all file systems waste space, as a simple example imagine your file requires 2.5bytes of data, ie 20bits, ie 2 full 8bit bytes and half the other byte, the 4 remaining bits of the 8 bit byte end up as slack on most file systems. Also consider JFSS etc mark bad bytes, so the actual capacity of the FLASH is less than specified. Not a definitive answer but pointers for further research.