It was not enough. After installing successfully VmwAROS, I noticed I could use the emergency mode kernel only (well, at least now I am sure it is really useful), so I modified the menu.lst file as explained on the "quick and dirty user/reviewer guide" included in the /Docs directory to cut any unuseful entry and clean my early boot menu to the minimum indispensable. But I dind't want to disable my SATA controller at all, and at the next reboot I entered the BIOS again.
Here I found a SATA controller mode option, and I notice it was set to "IDE" by default. I changed it into AHCI, wondering that this option may create some conflicts with AROS ata.device... and I was right: at the next reboot, I had still to use the emergency kernel, but my discs and partition were detected correctly.
In the end: if you can't access your ATA device, enter the BIOS and set the SATA controller to run as "AHCI" instead of "IDE", and if it's not enough, just disable it.