Struggling to install linux with LSI MEGARAID onboard card

After useless tries to disable it via BIOS, I figured out how to disable / bypass the RAID configuration.

The scenario:

• Acer Altos server with legacy LSI MEGARAID onboard card
• only one previous HD left, so I basically don't need RAID anymore

The problems are 3:

• the BIOS settings, to be setup from RAID to ACPI 
• the RAID configurations left on the HD from previous installations
• Ubuntu actually do recognizes the card even if disabled and refuses to use ACPI configuration

I solved this way:

• BIOS settings in Advanced / IDE configuration: disabled everything and enabled legacy support (it eats 2 SATA ports but I dont think it's mandatory... however I don't need them)
• from Ubuntu netinstall 16.04 I launched the command line (there's the option during the installation when the bootloader fails or you exit partitioning or choose safe mode at the beginning) and gave these command:
# dmraid -r
/dev/sdb: ddf1, ".ddf1_disks", GROUP, ok, 124914352 sectors, data@ 0
• take note of "124914352 sectors" and give
# mdadm --zero-superblock /dev/sdb
and then try again # dmraid -r
it should reply with no raid disks and with names: "/dev/sdb"
If the first gave error, get the sectors you noted before and give
# dd if=/dev/zero of=/dev/sdb bs=512 seek=124914352
and then try again # dmraid -r
• Now proceed to install CentOS 7, because Ubuntu will keep recognizing the RAID onboard card and refuses to use ACPI configuration