What usb stick can you use for HDMU

[rron]

Hello,
I'm already using the HDMU image. This image i have installed on a 1Gb stick. But no i'm trying to install the image on a larger stick but i have trouble mounting the usb. I have tried 4 different usb sticks but no luck. If I use the command fdisk -l i can see the usb but i'm unable to mount it.
What usb i have to use?
I am using the latest HDMU image and I have a ipbox9000hd.
Thx