T7920 Hard drive error light

I bought a nice tower 7920 and it came with a PCIe SSD drive as the boot drive. I added two SATA drives below that one and they work fine. When I put a SATA drive next to it, I get a flashing error on the drive "holder". So Dell swapped machines. The new one came with TWO SSD drives. I added my drives below and now THEY get the dreaded yellow flashing light. Help! I suspect it may be BIOS configuration. I turned off PCIe drives 2 and 3 (leaving 0 and 1 on). Didn't help. All the SATA drives are on.