I've been working with a Fast ATA MKII for about two years. It was liking to give me from time to time messages the kind of "Read attempt outside partition" or Wrong dirblock ID", when not just showing me the insert disk screen (this last is just when your connection with the kickstarts isn't good). What I've done, but only from two days ago, so I can't tell for sure it's a good solution is to disable the s-s call for the Fast Ata. Instead I've made an script that I've put with the same commands on the WBStartup. So, the Amiga boots without Fast Ata (when booting was when it was liking to give me the problematic messages), and when charging the WBStartup it starts the Fast Ata prefs. I believe to remember someone -or just the instructions- saying their commands should be at the beginning of the s-s, but all I can tell is that it works from the WBStartup, as I can see it working with SysInfo.
But there is another problem; at least with my MKII drivers. They will show the Fast ATA prefs as "Split", no matter that you use and save them as "Non Split".
I bought an MKIV this week -that's why I'm also interesed on this thread-. I want to try the new drivers and hardware.