I've tried the following USB drives with my Ez-Guide 500, and none of them work

Verbatum 512Mb
Sandisk Cruzer 1GB
generic 256MB

Whenever I put any of them in my EZ-Guide 500, I get the following error:

USB drive is not compatiable and cannot be used.  For a list of compatabile devices, please consult documentation. 

Does anyone know what I am doing wrong?  If you could share what brand/size of USB device you have successfully used with Ez-Guide 500, I would sure appreciate it.  Am I formatting the USB drive incorrectly??  I don't want to keep buying drives that won't work.

TIA!