ok perfect!
Once you have installed the whole AOS4, you can use your buffered interface.
It "seems" that the problem arise only during the installation (on some machines)
Theres a option to enable it.
look at the file SYS:Kickstart/KickLayout-A1200 at the line
; The IDE scsi.device can not autodetect IDEFix/IDEFix97/IDEFix Express
; IDE port doubler hardware. When you have such hardware installed,
; uncomment the following line by removing the leading semicolon,
; then the IDE scsi.device can use IDE drives on the second IDE port.
; CAUTION: Dont forget to add the semicolon again before removing the
; IDEFix or compatible IDE port doubler hardware or your machine may
; not boot anymore!
;
;MODULE Kickstart/A1200/ATAPIismajik
uncomment "MODULE Kickstart/A1200/ATAPIismajik" and it "should" work.