...and keeping in mind that the USB3 port supplies a MAX 900mA of current. This and the device draw is monitored by the system, as well as the amount of available or supplied current based on a few variables. Not all HDD's (or
ext. HDD kits of same model number, as the internal drive is a variable commodity) will draw the same current, and or when on 'spin-up' and i/o. So..... again... a non self-powered hdd may demand too much, nor get the required current to run.
Until I see evidence to suggest otherwise, I am sticking with the hypothesis that the 'USB Selective Suspend' setting was suspending power to the port. Your hypothesis, that the drives require more power than the port can supply, is falsified by the evidence.
Two different 2.5" external drives failed to work with the setting 'enabled'. The same two drives work perfectly with the setting 'disabled'. If the problem was that the drives require more current than the port can supply then they would not work on either
setting. Clearly it can supply enough power because it does supply enough power.
This, of course, is very good news and largely resolves the basis of my rant from a few days ago. There are still some disappointing issues with the Surface, but this one would have been a deal-breaker.