Since you have this motherboard:

http://www.asrock.com/mb/Intel/Z77%20Extreme6/?cat=Specifications

You should have those slots:

- 2 x PCI Express 3.0 x16 slots (PCIE2/PCIE4: single at x16 (PCIE2) / x8 (PCIE4) or dual at x8/x8 mode)
- 1 x PCI Express 2.0 x16 slot (PCIE5: x4 mode)
- 1 x PCI Express 2.0 x1 slot

From my perspective (having done PCIe signal quality measurements etc. already) any kind of extender cable are potentially trouble for both signal quality and power supply. Not being detected at boot time sounds a bit more like the former. But fortunately for bitcoin mining you don't need much bandwidth. So here is my suggestion:

Check in BIOS Setup (or rather UEFI Firmware Setup here) if you can set the PCIe mode to the slowest Gen1 speed. This should at least give you the highest chance of success, with no impact at all on mining speeds.