I've had several read/write errors also before the new firmware, now they seam gone but i must say i don't trust it.
The internal memory is actually an sd card, it's rather easy to open the reader with standard plastic spudgers
https://wxcafe.net/posts/pocketbook_...mbly_and_root/
just start at the side opposit of the buttons with a plastic tool, for the button side i used a steel spudger (smaller).
The sd card slots out, simply remove the kapton tape, push it in and move towards the battery.
i have pulled an image ofthe current 4gb sd card in my reader and wrote it to a 16 gb card - don't have any 4gb for the moment - using ubuntu "disks" tools - you should be able to write it with windows tools also.
I tried to extended the last partition to fill the 16gb card but was unable to do so because of an overlapping partitions error....
it also dit NOT booted with the 16gb card "as is".
so I'm now ordering a 4gb replacement card, will take bit of time before i can test...
I can share the image with you if you want , it's a blank factory reset image.
Just need an extra 4gb sd card to try it with.
edit: bloody hell, they charge more for a 4gb card than a 16gb card...