To add to the others:
It is easy

to damage the file system on the SD Card by disconnecting just a little too soon or just things that go *bump*.
Connect the device to your computer and run a "Check disk for errors" on the drive for the SDcard, tick Fix errors (DO Not have Calibre or another book manager running when you do this, the device will be
busy and want to
schedule)

I used the drive "Properties" to label my SD Card and the device Storage to make it easy to spot (PEz and Books1) in file browsers