Manual partitioning, no grub install

Hello everyone, I would like to test Calculate Linux on my machine (legacy bios, secure boot OFF)
I have the latest ISO image (Cinnamon), but when I try to install it with manual partitioning, GRUB does not install.
If I choose automatic partitioning, GRUB installs correctly, but it’s impossible with manual partitioning.
My partitioning (2 hard drives):

/dev/sda1 (win)
/dev/sdb1 (everything for “/”, no swap or home partition, etc.)

Is there any trick to install GRUB with this kind of partitioning?
Thank you!

legacy bios read 1st 64 bytes from /dev/sda and don’t understand what boot need from /dev/sdb1.
You need /dev/sda2 (fat32) (200-300 MB) and mount it to /boot when install.

It’s so.