[jshzh]

Thank you a lot, Alan_S, for the tip. Do you think I need to remove the index file in Nickle? I have over 100G books on the external MicroSD card. I wonder the index file will slow down the device under Nickle even after I moved all books to a hidden folder on the card.

For those who don't know how to set the Koreader to show hidden folders:
1. Click the top of the Koreader File Browser (not inside a book) after Koreader is launched
2. Click Tools and then Advanced Settings (may on the second page)
3. Scroll or turn pages to find DSHOWHIDDENFILES
4. Click it and change the value to "true"
5. Reboot Koreader, a pop-up window will ask you to save the unsaved default setting
6. Reboot the device to launch Koreader, reset the home directory if needed, clear history of deleted files (not really deleted) by long pressing any book.