I have UVO in a Soul, and I leave a USB flash drive in the slot all the time, I try to remember to turn off the unit before turning off the engine as it does say in the manual that this could cause a problem to your flash drive, but sometimes I forget but still have not had any problem.
When I start up the engine and its on or I turn the unit on and select it, it just continues from where I left off.
It only reloads the tracks when you remove it and reinstall.
I know from other problems I've read on various forums that there is less problems with well know brands, than some of these generic types. The brand I'm using is Lexar and its a 8 gb micro-flash drive also used a large 16 gb Kingston with no problem.
More than likely its usually the particular flash-drive, try another one or perhaps its comes loose in the slot. You can also reset the UVO system if you have problems that sometimes can correct it.