I got the identical Problem last week,
internal system microSD had turned readOnly.
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Reading the various hacky solutions here was rather bewildering.
So i wrote to
service@pocketbook-int.ru and asked for a new firmwareImage for a new microSD-Card.
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They need the DeviceSerialNumber, the SDSN of the new microSD and a picture of the sticker found inside the device on the PCB.
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After sending these three Informations and 5€ via Paypal, i could download the new customized fwImage.
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Next i used androidApp etchDroid to write the image to the formatted new microSD.
Inserted the Card in the cardHolder on the PCB and switched on.
Next the Device started to flash and partition itself during ca. 3m.
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Now everything works as before, plus an extra internal MemoryBoost due to the 32gb SystemCard.
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Conclusion :
Better not bother with the tricky Linux Hacks,
and instead get a new customFW from PocketbookRU directly