Hi once again,
In any case, you can try to brute-force wipe FS/partition/LVM info on the disk array by executing (data on /dev/sda will be unusable afterwards)
# dd if=/dev/zero of=/dev/sda bs=1M count=1024
and then try to reboot into the installer.
We've already deleted the RAID array earlier from these drives, so I suppose there is no harm in writing zeros to the whole thing. We'll try this next and update.
No luck. Wrote zeros to the entire OS disk array and still there is no unused disk detected in the installer.
Best, Dave