Here are some step you can take if you have some problems with booting your system or the card are not detected and so on.
- Flash the latest BIOS. This is a must actually.
- Sometimes CMOS reset help.
- Set PCI Express mode GEN1 or GEN2, try different.
- ACPI settings may help, turn it on or off.
- Try to start with internal GPU, then add 1 external in 16x slot, then add others one by one.