What if the device is broken? No matter how safe it is. Theres still a hole that can be penetrated. But its a good idea tho. Instead you store it to usb. Why not use more secure device which ledger. I heard about ledger nano. And they said its really good hardware wallet. I think its more secure than usb.
You can keep copies in multiple devices. Dont store in one device. Store 1 copy in your personal computer, 1 copy in Pendrive, 1 copy in Micro SD card, 1 copy in External Hard disc .. etc ...