Sorry Proper(S)!  How cool is it to refer a poster to their own thread?  One point I will make is in a very few weeks Threadripper II will be released and all the MB vendors will release new UEFIs.  Some have already.  GIGABYTE has released UEFI F10 with AGESA 1.1.0.0.  I think ASRock did also but then removed it.  That start/stop stuff may be memory training.  If you will explain the "weird" boot stuff detail, I will think on it.  I would prefer you use AMD forum, but this is OK if you prefer.  Enjoy, John.

ProperS,  thanks for the reply.  Here is the link to a thread with a similar experience as your's: https://community.amd.com/message/2864370#comment-2864370.  The user believes and I agree that the Above 4 GB option would allow use of all GPUs that are boot-able.  I assume your MB does not support this option.  What board are you using?  Hopefully a UEFI update will correct this soon.  Threadripper II is rumored to be released next month (August 2018).  Perhaps that will spark some new UEFI updates.  Thanks and enjoy, John.

Thanks, remauto1187ma.  Good advice.  I will keep it in mind.  Enjoy, John.

Thanks, remauto1187ma.  I will be more careful.  Please tell us how to increase the number of PCIe devices allowed by the UEFI.  To date, the only method known is to Disable all the devices not needed.  One real problem is that some MBs do not allow most devices to be disabled.  Perhaps you can tell us how to do this or recommend what to ask the MB Vendor to implement.  Thanks and enjoy, John.

Thanks much for all responses!  Here is a screenshot from a different user using an ASRock AMD MB.

This user reported that the setting did not work.  ASUS does not seem interested.  Thanks and enjoy, John.

EDIT: The video above is from an Intel MB, not AMD. 

Thanks, nitrobg.  By disabling as much PCIe stuff (USB Root controllers, NICs, etc) the ASUS board will post with 16 GPUs but only 12 are recognized by W10.  The MB does not have "Above 4GB" setting in UEFI.  Perhaps all 16 could be used if available.  Thanks all for the help.  I learned a lot.  Enjoy, John.

Thanks, remauto1187ma.  Does the UEFI/BIOS give the user access to the "Above 4GB MMIO"?  Thanks and enjoy, John.

Thanks, Raggie.  I could not find the sub-forum you recommended in the Move Thread pull down list, so I placed it here.  I do hope someone will help here.  Thanks and enjoy, John.

Thanks, adaseb, but I am looking at a mining type system with 10 to 15 GPUs.  This system is not being used for mining but other intensive computational work.  The UEFI will not POST if some limit of devices is exceeded.  I am looking for what that limit is and how to expand it.  I have tried to help others that were building mining rigs and ran into the same problem, apparently.  I hope some of you will be able to tell me how to circumvent this limit.  Thanks and enjoy, John.

It appears that for newer MBs there is an MMIO memory limit that is usually 4 GB which can limit the number of PCIe cards (eg, GPUs).  There also appears that even if this limit is increased the system may not POST.  By disabling some unused PCIe devices the system POSTs.  This seems to me to be a UEFI limit on how many peripherals can be handled regardless of the MMIO space available.  Right now the system I am talking about is the AMD X399 Threadripper and several MB vendors (ASRock, ASUS, etc).  As you can tell I am not very knowledgeable here and any advice and information would help very much.  Thanks much and enjoy, John.