ok to make this a little easier, i created a patch with the object embedded and already filled with data.
just take the patch live, once with append set to no once with append set to yes.
lpcwizardwithdata.axp (12.6 KB)
and here is a version with the same data doubled. take this patch live (only once) and the data should be identical to the first version. however they are not. only the second one is ok. you can check this easily with a hex editor...
lpcwizardwithdatadoubled.axp (20.6 KB)
since the data is written correctly with the second version, i think there is a bug with f_lseek on axoloti/chibios? this happens with two sdcards i have tried.