When I drained the coolant through the drain plug on the engine block, I did it twice: once when I changed to Evans, then a couple months later when I reverted back to the OEM blue. This was on my e39. It was not hard, very easy accessible. To drain EVERYTHING you have to also take off the hoses that go to the firewall into the heater core. there is a fair amount of coolant in there as well. I also used compressed air at lower pressure settings.
the drain plug of the radiator will NOT drain what you have in the engine, only what's in the radiator.
Also, through the WP/T-stat opening, not everything will be flushed out.
Through the engine drain block, I collected an extra 1/2 gallon or so, after the WP/T-stat was out, and the radiator plug was open.

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Stable: e92is, e46 M54B25, e83 N52, e53 N62 - sold, e39 M54B30 R.I.P.