Good news:
9atom successfully detected the SATA drive, the unpartitioned space in it and used it for the installation.
Bad news:
It ended up corrupting my Linux root partition which was after the unpartitioned space. Fortunately I had a back of the root partition and I could quickly recover it. Having some issues booting (rootfs is being mounted ro). Once I fix this I'll check if the 9atom installation is broken because it might be assuming it has some space in the next partition. I'll let you know how it goes.