For anybody who is looking to add a USB-port to their D8B or HDR, this part works (at least on the new board, but I think I also had this installed on the old board at some point... but I'm no longer certain... too long ago):
https://www.ebay.com/itm/334461327689
And for $10 new, you really can't complain, IMO.
I already had a single Asus USB 1.1 board exactly like that (that I don't recall what other computer that came with, originally) in the D8B rack unit I turned into a Win98 computer, and it works great for that.
When I ran across that eBay offering, I bought another one and swapped it with the one in my Win98 D8B computer to make sure it works before ordering more.. and the new board works just like my old board. So, this is confirmed to work
I think that theoretically "any" USB 1 (or 1.1) expansion should work... but sometimes it's not clear from the product listing, and I bought two similar USB expansion boards before, that did NOT work with the D8B (probably wired for USB2, or some other pinout for the connector on the motherboard). I didn't want to "customize" the wiring of these other USB extenders I had ordered and was just waiting for a drop-in "as-is" solution to come along, and the link above seems to be it.
I just placed an order for another 2 of these ASUS USB/MIR boards, so I can keep one in each of my spares and/or use these D8B computers for additional vintage PC projects (i.e. my Win98 D8B computer exists to house my Ensoniq Soundscape Elite card (the soundcard I originally started writing/recording music with)... and is convenient for loading/converting old files of mine from back in the day). I can always remove that stuff again, if I'd need the spare for a D8B-mixer setup.
Besides, I have another ISA soundcard I can't use in the same converted D8B computer at the same time because of IRQ limitations (Terratec EWS64S - needs the same IRQs as the currently installed Ensoniq Soundscape Elite card), so I might just build another Win98 D8B computer for that from one of my spares (...although that card doesn't really have anything on it, that can't easily be recreated with software... the Soundscape card has a ASR/TS type FX chip on it (ESP/OTTO).
If anyone would like to explore the idea to run Win98SE on a D8B computer and use USB... you have to install an additional driver to make it recognize USB thumb drives/memory sticks. (NUSB 3.6).
When the USB port is in use with the D8B running the Mackie OS, what it can do with that port is rather limited. I don't remember off hand, but it either only supports a USB keyboard OR a USB mouse (I forgot, since I had used the only USB-expansion board that works with the D8B hardware, with that Win98 box, instead... but there is no USB driver for either one of those built into the Mackie OS... so, only one of them will work... forgot which. And I'd think that USB memory sticks likely also wouldn't be recognized when running the Mackie OS on the rack hardware... so, added functionality is pretty limited... but still - I looked for these expansion boards before a few years ago and couldn't find any. So, load up on new such boards while they're available, if you were thinking about expanding your D8B with USB ports
If you have a spare D8B computer and feel like turning it into a vintage Windows system, either to support old hardware (e.g. ISA cards) or as a vintage gaming system to play DOS-games, then you might want to look at this page... much of that stuff is useful, when bringing an old Win98 computer back in service, nowadays:
http://dk.toastednet.org/vogons/win98/
...or generally, the "Vogons" forum is good for that:
https://www.vogons.org/
(besides, Vogon poetry is the BEST!)
...oh, and just a thought: If your D8B no longer works as a mixer and your only using it with a ProBox, you could as well install Windows on the computer side of things and run that while using the D8B console with the ProBox. The rack computer literally does nothing when used with the ProBox, other than provide power. Might as well have it run Windows (or Linux) and be available for conversions or little test projects etc.
...and for that, the USB expansion board I'm linking to above, is really useful, IMO