Not all boxes support flashing from ports at the back, they're of a different type on a different controller.
Most do however have detection issues if there is another USB device connected, so make sure they're unplugged. I use a 30cm USB extension lead for that purpose, so I don't have to remove the box from its built-in spot to get the stick out of the back.
Also, the only two requirements for the stick are:
- formatted in FAT
- fast enough to be detected
The second point is why some USB sticks fail. The bootloader has a short timeout in order to minimise the boot delay, Size of the stick plays a role, but not a definitive one, I have 8GB sticks that work, and 128MB sticks that don't.
And it is fine to have other data on the stick, I usually have images for all boxes on my test bench on the same stick, together with a "tuned" backup I restore after flashing a test box.
Currently in use: VU+ Duo 4K (2xFBC S2), VU+ Solo 4K (1xFBC S2), uClan Usytm 4K Ultimate (S2+T2), Octagon SF8008 (S2+T2), Zgemma H9.2H (S2+T2)
Due to my bad health, I will not be very active at times and may be slow to respond. I will not read the forum or PM on a regular basis.
Many answers to your question can be found in our new and improved wiki.