hi again eric.
you can't do a "chip erase" with isp(flash magic).
only block 0 erase(user code).
You can erase anything but not
security bit (if it is set).
And if security bit is set then you have no more access to flash if you want to update your program.
So if you want to work with ISP you must leave security bit unset and your code open to a paraller programmer.
It looks strange.

lambros.