|You don't normally connect a USB hard drive to a Modem. A Modem has one input, either a telephone or cable, and one output, usually Ethernet or USB.|
If you have a device with a USB input it is probably a Router with a built in Modem, perhaps not. Some do take a hard disk, some don't. The USB might be an alternative to Ethernet.
Without a Model No of the device you are trying to plug this disk into we could be here guessing all day and all night as to whether it would work or not.