Backup all important data first!
Create a mount point and mount the usb flash drive:
It'll still read-only, so execute following commands:
voilà, now you have a write access to the flash drive, but you shouldn't use it for any important information anymore!
Rneeded = (Ufan/Uwished - 1) * Ufan / Ifan
i.e. for a fan with parameters 12V / 0,13A and wished voltage of 8V you need a resistor with 46 Ohm
R = (12/8 - 1) * 12/0,13 = ~46 Ohm