I tried several different USB cables, didn't work. I made sure they were 2.0 as well. I actually got in touch with Digital Igloo via The Gear Page where we're both members. He gave kindly gave me some suggestions, all of which I'd already tried unfortunately. Still, he took time over a holiday weekend to try to help me, and I appreciate it.
However, here's the good news....I finally got it working.
Uninstalled everything Line 6 related from my computer....Monkey, Edit, drivers, Lic Mgr.
Reinstalled everything, manually by downloading direct from the L6 site.
Downloaded the current flash update from the website directly to my computer.
Instead of letting Monkey do the install as it usually would, there's the option to "Install from file" button down on the lower right of Monkey. This lets you take the downloaded file from your own computer instead of installing directly through Monkey and the L6 server.
For whatever reason, that worked. Same computer, USB cable, port etc. Finicky stuff.
Just for kicks....I went in again and hit "reinstall" for flash and let it try to do it from Monkey. Again, went haywire and knocked my HD500X into the frozen mode it had been in before. Went back, installed it directly from the file again...and once again, it worked. This tells me its simply not some USB issue but rather some kind of problem between Monkey and the Line 6 server its trying to get the update from. I've seen mention of users saying its a firewall issue with some pc's and anti virus software. I never bothered turning any of mine off, and frankly don't feel like I should have to for something like this. Whatever the case, my HD500X is now updated and operational.