Something is going on at the BIOS level with USB 2.0 devices causing many of them not to work. I didn't change settings when this started to happen, and reverting to motherboard defaults does not fix it. I have reflashed BIOS, chipset driver, and Intel RST driver, none of that helps. With respect to USB 2 storage devices (USB flash drives and hard drives) this extends all the way down to BIOS operations without Windows booted at all.
 
Here's the overview:
 
USB 3 devices: work fine wherever plugged in, including all motherboard ports and the USB 3.0 hub on my desktop.
USB 2 keyboard, mouse, game controller: work when plugged in to any motherboard port, including front panel. They do NOT work when plugged into the USB 3.0 hub, which is, in turn, plugged into a USB 3 port on the back panel. I have validated that the hub is good on other hardware; it's only here that it fails.
USB 2 storage devices and webcams: do NOT work, no matter how they're attached.
 
This DOES go all the way down to the BIOS level, in that USB 2 storage devices (in the form of a USB flash key) are not seen from BIOS, while USB 3 devices (also USB flash key) are. USB 3 devices can be read and written, as tested by storing and recalling profiles successfully from them. But USB 2 storage devices are simply invisible to BIOS, even as USB 2 mouse and keyboard continue to work just fine.
 
Again, resetting to factory settings does not fix this; re-updating firmware versions I know worked until October 22nd (which is when I first saw the problem) does not fix this.
 
Any ideas, anyone?
 
 

So having done everything reasonable at the motherboard level - I reburned BIOS in case something happened, I reset CMOS, I reinstalled the other motherboard firmware components (chipset drivers, storage drivers), I set factory default, etc etc, nothing worked...
 
...I started taking the system apart to see if I could figure out what's up. First thing I did was pull the USB 2 front panel connector (connected via USB2 header 1) and the USB 2 LED controller connector (connected via USB2 header 2) just to see what would happen. (The Corsair LED controller, while not previously mentioned, had been functioning correctly throughout this.)

Everything started working correctly on the USB 3.x ports. USB 2 mass storage, the camera, USB 2 devices plugged into the 3.0 hub? Everything. Working. Just fine.

So I'm thinking "okay, we've narrowed it down a lot more now."

I plug the LED controller back into USB2 header 2 and power back up.

Everything keeps working. "Well," I think, "I still have my fairie lights. Nice."

I examine USB2 header 1, and the front panel cable. Both look completely fine. They've been working perfectly on the devices that did work, as well. Everything's clean and shiny. So I plug it back in and power up to confirm that I've found the cause of failure.

Except I don't get failure. Everything keeps working.
 
It's fine now. All of the problems vanished the second I unplugged the front panel and LED controller from the USB2 headers, and they have, at least for the moment, stayed gone after plugging them back in. At least, for the moment.

I have no idea why. I built this machine 20 months ago, right before the plague. Those two things haven't changed since installation.
 
I'm at a loss, honestly. Anybody got any ideas? Because... what the hell. I mean, I'm glad stuff's working but having no idea why just makes me assume it'll blow up again at random for no reason.

Neither felt loose, and there's no obvious oxidation, but I mean, maybe? Everything looks shiny and nice.
 
Literally the only other thing I did was dust out the case with air. It had been a few months. (Not 20 months, it's a front-mesh case, I do that every so often for all the usual reasons xD )
 
Maybe the RGB controller was in some particularly bizarre state? But that's not where it gets its power and I'm kinda just grasping at straws. (Well, wait. It's not where the LEDs get their power. Maybe the controller gets its power there. So maybe that's less straw-graspy than I thought?)
 
I do have fast boot on, and legacy USB device support is on, and neither of those states changed when things stopped working - or when they started working again.
 
Still working, btw, as of early this morning. Which is great! But it's just so... why?

That's all well and good, and I'll certainly keep those in mind if (lol when) I encounter future issues, but this problem didn't even require booting. I could reproduce the issues while being purely in the BIOS screen.
 
For example, not being able to see USB 2 devices like keyboard and mouse when connected via the USB 3 hub. Also, not being able to see USB 2 mass storage devices even when formatted correctly, which I could verify this by save/restore of configuration to a FAT32 USB flash drive, for example. No external-to-motherboard software, no Windows at all.
 
So while those are both issues for me to watch out for in case of other problems, here, none of them can really apply since there's no boot process (and at that point, no Corsair software other than I guess LED controller firmware) involved.
 
This would be an easy problem if it hadn't started working again - some sort of motherboard USB controller issue, maybe time for an RMA - but it started working again after doing a very simple thing, so I'm... yeah.