SupportAssist won't recognize USB capacity in order to make recovery drive

I have a Dell 8700 desktop with Windows 10.  I have been trying to make a Recovery Drive through SupportAssist using  a 32GB SanDisk USB and also tried a 16GB Lexar USB.  Trying both USB's on all available USB ports, SupportAssist tells me that my USB is not the necessary 8GB. The USB ports all work when connected to an external hard drive, so the problem seems to be in recognizing the size of USB's.   Thus, I am unable to make a Recovery Drive.  Help please.