ljthey, Thank you for posting in the Intel® Communities Support.
As you mentioned, according to Gigabyte's web site the board does support the processor when using F10 BIOS version:
https://www.gigabyte.com/Motherboard/Z370-HD3-rev-10#support-cpu
The Intel® Core™ i9-9900K processor has 8 physical cores and 16 threads:
https://ark.intel.com/products/186605/Intel-Core-i9-9900K-Processor-16M-Cache-up-to-5-00-GHz-
Did you try to restores the BIOS settings to defaults?
Depending of the BIOS of the board, you might be able to see the quantity of cores the processor has, do you see the same amount of cores in there?
Please enter "task manager" in the Windows* search bar, then look for the tab that says "Performance" and verify the cores and logical processors showing in there.
Also, we can run the Intel® Processor diagnostics tool, once the test is completed you will see a report that shows the number of cores and threads:
https://downloadcenter.intel.com/download/19792/Intel-Processor-Diagnostic-Tool
Regards,
Alberto R.
Intel Customer Support Technician
Under Contract to Intel Corporation
