@Joe563 - All the books are in ePub format, none over 5 MB tops. I have tried just copying the files from my main Calibre Library folder on my hard drive and pasting them into the SD card. It worked on previous occasions but now it has mysteriously stopped working.
@PeterT - My SD card is in MS-DOS (FAT) format. Does this make a difference?
Thanks for all the ideas so far, appreciate it!