The standard Desktop installer nor gparted work with RAID systems. Either the alternative installer or the server installer are required.

You can add RAID drivers to the desktop so you can run repairs in a liveCD mode or do some things. There are many types of RAID, so you have to use the correct tools for your version of RAID to modify partitions.

If you are not really running RAID then you have to remove RAID meta-data from the drives. Then the Desktop installer will work as it is not RAID. But BIOS should not be in RAID unless you have RAID. It should be AHCI or Large or LBA as drive setting.

Generally with drives of two different sizes you are not running RAID although some new systems have Intel SRT which uses RAID to make a SSD work with a hard drive.

If you are sure you do not have RAID.

Presence1960 on remove old raid setting from HD
sudo dmraid -E -r /dev/sda
sudo dmraid -E -r /dev/sdb
Also check BIOS for raid settings