If you put something like this in a while (there are files that start w/ a) loop, you should get around the issue of too many arguments. (Be sure to set increment=1 before the loop). 
mkdir dir_a.$increment
mv `ls a* | head -n1000` dir_a.$increment/
let increment++

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In reply to Re^3: Finding and sorting files in massive file directory by frozenwithjoy
in thread Finding and sorting files in massive file directory by CColin

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