I've been struggling with a USB Ground loop problem in my set since I began this as a hobby last year.  I have a few synths as well as a Focusrite 18i20 Gen 2.  I've tried several suggestions from this forum (as well as the Keyboard Central forum), but nothing has worked.  My next step is to buy an inexpensive PCI to USB 2.0 card to plug the Focusrite into.
 
So, the question is this - does it matter of it is a PCI or PCI card? I can get a 4 port USB 2.0 PCI card for $11, or a 3 port PCIE card for $33. Would it really matter? I understand that I would not plug any of my synths into that, but would it hurt to plug in my Motu into the PCI (or PCIe) card?

With a 170 motherboard you most likely only have PCIe slots.  
I have a lot of USB devices and do not have any "ground loop" problems.  
 
What have you done to determine its a ground loop problem?  I've seen and worked on a lot of ground loop problems but never one blamed on USB.

I also have a USB ground loop problem with the Line 6 HD500. Both the USB and SPDIF connectors are grounded, thus the loop. Luckily, I only need the USB to setup patches so I can use it w/o the ground loop. Wish they used TOSLINK instead of SPDIF.
 
A ground loop requires TWO grounded devices for there to be a problem. You've sort of found one of the devices - your USB. But what's plugged into it, the Focusrite? The USB not the problem per se, but what's plugged into it. Then you also have to find the other culprit. What is it that's completing the loop? If you unplug everything except this one USB port and it still hums, then it's probably the power ground between the computer and what's plugged into the USB. 
 
Why do you think a PCI(e) card will fix the problem?

USB interfaces can be influenced by improper grounding of devices that are plugged into it but also by the PC power supply itself, even with self powered interfaces. What you hear is not the typical 50 or 60 Hz hum but higher frequency interference that is very annoying in the monitoring chain and also printed on recordings.

Years ago I suffered from these issues. That was before building dedicated treated rooms with power supply totally isolated from the rest of the building. I threw money at power conditioners, DI boxes, ferrit core USB cables but could not solve it. Interestingly once everything was hooked to a separate power line plus a power conditioner for safety the same PC and interface combination went totally quiet.

So it is not an easy thing to fix. I'd try with plugging everything into the same power socket, try different ones as you will have different circuits in your house and it does make a difference if your AC or washer/dryer are in the same circuit. Also try with nothing plugged into the interface and then add slowly to see if a particular synth, FX or likewise makes it worse.

It definitely is not an easy thing to fix - I posted and participated in a number of threads regarding ground loop problems here on on the Sonar and other forums, and I have tried all of the normal suggestions, but the problem remains.  Anderton (on another music forum) suggested a PCI card to USB and plug the interface into that.  At this point it is worth a shot.
 
My Mobo has the following slots per the specs.  I was surprised as well to see the old PCI listed there:
PCI Express 3.0 x161 x PCI Express x16 slot, running at x16 (PCIEX16)
* For optimum performance, if only one PCI Express graphics card is to be installed, be sure to install it in the PCIEX16 slot.
1 x PCI Express x16 slot, running at x4 (PCIEX4)PCI Express x12 x PCI Express x1 slots
(All of the PCI Express slots conform to PCI Express 3.0 standard.)PCI Slots2 x PCI Slots

The PCI slots on newer motherboard (e.g. Z170 boards) are not true PCI but bridged to the PCIe bus.  I have two PCIe five port USB 2.0 cards installed in my Z170 system.  Work perfectly.   

OK, thank you, that makes sense. This is what I was looking to get.

https://www.newegg.com/Pr...166014&ignorebbr=1

Just a thought, you may want to consider PCIe, since that is direct and not bridged.  Just in case.  That's because I'm superstitious when it comes to converters/adapters/bridges, etc. Also, your next PC might not even have PCI slots, so with PCIe, you could redeploy the card.

Not saying it won't work, but if you have issues, you may wonder.  I don't even like USB hubs.

I just add these cheap header cable brackets to my rear panel and connect them to an extra USB header supplied on my mobo.
 
Like this one:  https://www.newegg.com/Product/Product.aspx?Item=9SIAE4P5VW2097&cm_re=usb_bracket-_-9SIAE4P5VW2097-_-Product

I'm not a Rosewill fan.  I've had some of their stuff and it was all cheaply made or didn't last.  Rosewill is newegg's house brand.  
 
This is what I'm using for one of my 5 port cards
https://www.amazon.com/Sy...ords=pcie+usb+2.0+card

Thanks guys, I ended up getting this:
 
https://www.newegg.com/Pr...x?Item=N82E16815158172