Problem with bios boot during installation

[micdup]

Hello,

I have an imac on which three operating systems are currently
installed: max os, and two linux.

I want to replace one of my linux by the centos 7 linux distribution.

So booting from a livecd I followed the installation procedure
up to the manual partioning step. At that point, I deleted a linux
partition, created a new one for the centos linux (standard partition ,/, ext4).
When I was done I clicked in the finish icon and I was asked to create a bios boot
partition because the BIOS needs a special partition to boot from
a GPT label. The reason is that (I am trying to translate ...): "sdb may not have
enough space for grub2 to integrate core.img when using the file system
ext4 on partition".
Is it normal ?

Even if I create this bios boot partition, 2MB large (what seem to be the max
size allowed), it is not working and for the same reason: "sdb may not ...".
I don't not much about operating system.
I would really appreciate some help.
Thanks