I am surprised, too. Pretty much a straight shot from the battery. The disconnect thingy and then 10-12 feet of heavy wire. Sort of good I think - points to a not-subtle problem. Loose terminal or bad disconnect.

When the car does not start, what is the voltage drop from the battery safety terminal to the jump start terminal?

I have not competed the task of, once again, getting to the battery to check voltage and then also past safety terminal this particular time. I have removed everything except the pump over the battery. The current voltage at the jump start terminals is zero. The few other times this has happened the voltage at the battery and past the safety terminal was 12+ and zero at the jump terminals.

Plan is to start fresh in the morning and keep at it until I find the problem. Extremely frustrating when I can't make it happen and when it does happen the fixes have not been the same and none permanent.

Appreciate all input.

I think you might be getting fooled by a bad ground. What are you using for ground when you are testing ? If you could put a light or buzzer on the boost terminal before you start working your way to the battery it might be interesting to see if/when it comes on.

I think that sounds the most likely cause.
My issue was different in that I still had power elsewhere.
A bad ground would explain intermittent no power.

bad ground-not the problem
BST-not the problem
ignition switch-not the problem
There were no codes when I checked
No parasitic draw

Recapping, battery was dead one morning. Load tested it and it was bad. Replaced the battery. Tested as good before install. All was OK for that day. Next morning it started fine but wouldn't start at the alignment shop a short drive later. There were no dash or interior lights-nothing. Jump started it and drove it home. Turned it off and it started fine. Next morning it wouldn't start. Battery voltage was 9 something. I charged the battery for several hours. Turned on key and dash and interior lights and angel eyes came on normally. Tried to start the X and wouldn't even click and all lights either went out or were very dim. That was when I asked for suggestions as to what other than the battery could be the problem.

After eliminating all the suggestions I decided to charge it again. I ordered a battery and electrical system tester so I could thoroughly check the battery rather than just a load test. The battery failed. So back I go for another battery. They started by telling me it had to be something other than the battery but their test came up a bad battery. They brought me a 3rd battery and called the manager over to approve the exchange. He said the battery shouldn't have been sold to me as it won't handle the overnight electronic draw of a BMW. Not buying as that would not ruin the battery in two days. The battery exceeded the BMW battery specs and it comes up as a BMW replacement option.

Anyway, it has been 3 days with 3rd battery, the one he recommended and everything is back to normal. Problem solved.

When it won't start there is no voltage under the hood but voltage at the battery is normal.

That has never been the norm in my experience. The voltage at both places has always read the same.

Haha, whoops, Sorry I didn't put the quotes around it! ;)

Yes exactly providing the battery is good. If the battery is not good you may not have any voltage up under the hood. This is how it is on my X and 540It