Can I put personal files (music etc) onto the usb after I put the ISO onto it with the program?
If you're wanting to install Windows from a USB drive, then you'll want to use this with the ISO, instead of IMGBurn:
Microsoft Store
I would wait to put personal things on the flash drive, because that tool will format the drive, then extract the Windows files on it for it to install from.
After you get Windows reinstalled, then you can delete all the content (or format it, whichever) and put whatever you want on there again.
If you're needing the flash drive to back up personal files from your current install... then you'll want to either: a) get another flash drive, b) burn the ISO to a DVD with IMGBurn, or option c) would be to be what's called an "in-place upgrade".
On the install options, if you select your hard drive without formatting it first, and click Next to install, it will say that it found a previous version of Windows, and it will move everything from the old install into a "Windows.old" folder on your C: drive. If you do that, then you don't necessarily need to back up your files because you can just go into that Windows.old folder, and move your music/pictures/documents/etc over to the new Windows directory. Note: you CANNOT transfer installed programs.
After you moved everything over, you can run a Disk Cleanup, and it will remove the Windows.old folder to free up space.
Now, remember, you'll only need to reinstall Windows if you get a BSOD after switching motherboards. If you don't get a BSOD, you don't need to reinstall.