An external USB will only be upto 16Gb regardless of how big the USB stick is. This is also the same for external HDD devices.
The only device that will function properly is:
a) A proper HDD from MS
b) an adapted HDD from another 360 (videos on how to dismantle them are on Youtube)
It's worth noting that some games will give you problems (eg Halo Reach) when using a 4Gb slim as the 4Gb is not classed as a HDD on the console.
Cheers will halo reach work if say I purchased the Microsoft sandisk 16gb usb for the xbox
Cheers will halo reach work if say I purchased the Microsoft sandisk 16gb usb for the xbox