I'll just chime in, i'm a n00b but this might help you.
I had recently bought a premium that came with samsung ms25. I updated it to the latest recommended firmware: Xtreme42_Update
I flashed it fine with no problem, via my sata slot on my nforce4 chipset motherboard.
I updated the 360 with the fall update, then of course I get E66 error. DAMN!
So I read up and the new firmware has a batch file that will generate a firmware to fix it. So I do and go to flash it with the new one and I get the mtkflash errors. I read up a lot, did the trick where you power off/on, but that method was supposedly to fix bricked drives. I could not figure it out and no guide really said anything.
THEN, after many posts that I read, one guy said, that with the new firmwares, it locks it out from being flashed unless it goes to 0800 mode. DAMN! Yes it was that simple, so I put in the 0800 disc to put it in that mode, then bam, quick no problem firmware, no more E66.
Hope that helps, guides/writeups really need to make that clear, esp for n00bs. I went on a damn hunt.