Dear all, I am almost new to all this

Hope you can help, i have got a TM Twin OE for a while now. I have tried to record to a external device that connects with a USB cable, but every time i try it says unable to record because of the format. However i have used a Pc to format it but still unable?! can anyone help if i am not mistaken the size is less than 500 GB
Looking forward to hear from you!
cheers