I had a tough time figuring this out, a lot of the information I found online was outdated and/or misleading. After a lot of research and consulting with crypto mining rig experts I finally figured out the it was the bios settings. Here are a couple of links below that will point you in the right direction.

https://www.biostar.com.tw/app/en/support/faq_content.php?S_ID=776

https://www.biostar.com.tw/app/en/support/faq_content.php?S_ID=783

Before following the steps below to activate your 5th and 6th graphic cards/GPUs for mining cryptocurrency, make sure you’ve completed the initial troubleshooting steps:

• Check all risers and cables to ensure they are plugged in securely and are not faulty.
• Update the drivers for your GPUs to the latest version.
• Make sure the GPUs are plugged into the rig correctly and that they are not faulty.
• Check to make sure that your power supply unit(s) can handle the extra power needed to run the additional cards.

Step 1. Access the bios. If you have your rig hooked up to a monitor via the HDMI out port on one of your cards, this is the first mistake. Since the graphic cards are initialized/loaded after the bios and operating system, if you get into the bios you will not see anything on your monitor (because technically the gpu’s have not loaded yet). The way around this is to find the DVI-out port on your motherboard. If you are using the Biostar TB250-BTC you will see it in the corner next to the USB ports. You will need to connect to this port to display your Bios Control Panel on your monitor (I recommend using a DVI-HDMI adapter or cable). You will also need to unplug all your GPUs from the mother board so that the motherboard knows to use the DVI output (it’s the default if there are no other video output devices plugged into the motherboard.

Step 2. Once you have all of the above sorted out, reboot your rig by shutting it off completely, waiting 30 seconds, then turning it back on. As soon as you see any graphics on your monitor, hit the “DELETE” button on your keyboard and that should get you into the Bios Control Panel without any issues. If your board is a different model from the Biostar TB250-BTC, you may need to access it with some other keys (refer to your owners manual).

Step 3. Once you are in the Bios Control Panel, navigate to the “Advanced” tab and enable “Above 4G Decoding.” This will enable your motherboard to allocate the extra memory needed to run more than 4 cards from the motherboard. Make sure to “Save & Exit” by hitting F10.

Step 4. While the rig is still off, plug your GPUs back into the motherboard 1 at a time. Try to make sure to use the same slots as before so there are no unexpected issues (labelling them helps).

Step 5. Power up your rig. You should now see your 5th and 6th cards loading successfully.