Just for reference, the SD card is used for kepubs when the main memory is almost full. I did some experiments a while ago to test, but I don't remember exactly what space it was. I think it was at 100MB free. I think it is making sure there is enough space for updating the database during use and to download and unpack a firmware update.
Also, if you free the space, the kepubs are not moved back. But, any future downloads will go to the main memory.
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