use cable box external drive for time machine backups

I have a cable box that has usb ports. I plugged in an external hdd to one of the usb ports and I can access it from my mac book pro (yosemite) using ftp.

I would like to use this drive for my time machine backups since I don't want to plug in my hdd to my laptop every few days.

I tried transmit to mount the drive but time machine doesn't recognize it as an external drive.

Any way to make this work?