Downgrading a 3.56 minimum firmware question.

Heya,

I've done a few downgrades / CFW installs using an E3 flasher. Never had any problems.
This is the first PS3 I got that has 3.56 minimum though, and I have some problems with it.

I did as I always do, to start with. Take 3 dumps. Verify them all. Patch one of them and write it back.
Then I read that you should never install a 3.55 firmware on it, since that will brick it. Instead, just go to a CFW version 4 directly. I figured that was fine, so I tried Rebug 4.76.1 from recovery menu. It loads the update fine and displays the title fine, but then says the crappy "The data is corrupted. (8002F325)". Huh?

Granted, after I wrote the patched firmware back to it, I did not read it again to verify...

But, this should work, yes? Something must have gone wrong when writing it back, or am I way off here? Should I try writing the patched firmware again? Right now its on (patched) 4.75 OFW

Thanks!