I only ever use Kingston USB pen drives for flashing, a cheap 4GB is all you need, never had any problem with them.
Anyway here is a simple step by step guide to flashing the ZGemma Star 2S for those who have trouble:
01: Create a folder on your desktop called "ZGemma"
02: Download the image of your choice and save it in the "ZGemma" folder you created
03: Right click the zipped file and select "Extract Here"
04: After extracting you should have a folder that says "zgemma" (forget the other files if there are any)
05: Format a USB pen drive in "Fat32" ( I prefer to uncheck the quick format box)
06: Go back to the "zgemma" folder, right click and select "Copy"
07: Go to your USB pen drive, right click and select "Paste". The "zgemma" folder will now appear on the USB pen drive
08: Switch of the power at the back of the ZGemma Star 2S (white button)
09: Insert the USB pen drive in the "REAR" USB port (it will not work from the front USB port)
10: Switch on the power to the ZGemma Star 2S
11: You should see the letters "FLSH" on the front display panel (do not press any buttons until prompted)
That's it, hope this helps.