Hey there, Guerreiro.
This is a bit weird. I'd recommend that you try reformatting them with the flash drive manufacturer's formatting tool (most of the manufacturer's offer such tools for their memory cards and flash drives), to see if the same thing happens. You could also try the NTFS file system, to see if the same thing happens. Try the flash drive with a different device or USB ports as well.
Hope that helps. Please let me know how it goes.
Boogieman_WD