Os change doesn't work

Hi there,
I own the Mpc1000Bk with jj os1 4.99l on it. today i changed the os back to V.1.07 to try out the jessiah glitch. The os installation worked well. now i want change back to jjos1 4.99l, but if i hit the load screen and pick out any os file it won't load it... seems like the do it button F6 doesn't work. but if i want to load a sample or program F6 works perfectly!
What the hell is wrong? Need Help

can it be, that the mpc1000bk doesnt support akais os v1.07?

btw: they are the right .bin files, not destructed or invalid

ok i figured it out now:
i had to turn power on while holding down the shift button to reach the "os install mode" after that i had to push the rec button and the installation started...