You do not need to "erase before write", you need to "erase before write to a previouskly written cell"


an arased flash cell is always 0xff (if inverted it will be 0x00).

set up a struct
mark
... your actual data
...
...

then

for write
read, if mark = erased - write here and change mark
if mark != erased read next struct
if at end of flash - erase and start over

for read
if mark != erased - next struct
if mark = erased previous struct is the one

Erik