I have found that if I only use internal memory without external memory card inserted, my 500 is much more responsive, especially for bookmarking and menu navigation.
Since we already have the "delete book hack", accessing to the file system of internal/external memory is possible. Is it feasible to create a "copy file hack" to copy file between internal and external memory? We can make it using similar control as "delete book hack": press "SIZE" to start copy and press "MARK" to confirm.
Also, a hack to display available space for internal memory would also be very useful in addition to the copy file hack.
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Update: I have implemented it myself. See
https://www.mobileread.com/forums/showthread.php?t=14855