Hi maillet!
Chances are that the problem is not caused by the USB flash drive itself, since it sounds like the problem occurred right after you have upgraded your mobo. Anyway, I'd suggest that you try it with a different computer/device just to make sure about that. I'd also recommend that you test a different USB flash drive or an external HDD if possible, to see if the problem reoccurs.
You should also check the Power Options and more specifically - your Power Plan Settings. You can access the Power Options through the Control Panel. Once opened, click on "Change Plan Settings" for the currently selected power plan, then "Change advanced power settings". There you should look for "USB Settings". Click on the + symbol to expand the menu and do the same for "USB Selective Suspend Setting". If this is set to "Enabled" change it to "Disabled" and click OK. After that try the flash drive, to see if that has fixed the issue.
Hope that helps. Please let me know how it goes.
Boogieman_WD