[yencabulator]

SSD/NVMe keep track of what regions are wiped and which contain data that has to be preserved. To hide something in the seemingly-unused space, you have to turn off trim, eat the performance cost, and pretend you had a reason to have turned off trim.

[teravor]

I don't believe having trim disabled even helps here. smart firmware sees the same address being written to and may therefore reassign it to a different cell for wear leveling. it's a de facto trim.

trim lets the firmware know which mappings it can discard without the explicit reuse of the same address.

however I don't believe you can observe this effect from trim command results, it will report the usual size trimmed as if the firmware never realized that you reused the same address range multiple times.

[yencabulator]

Data recovery providers can probably get access to firmware-internal metadata.