Hi there,
It seems that in fact, there were too many books on my reader. I do not
know what 'finder' is and a google search was unhelpful, however, I went to
the guide on my reader and followed the instructions there using My
computer.
It would have been helpful if ADE could have told me that my ereader was
full! but at least, if it happens again, I shall start with removing some
books
Thanks for your time