You'd need an external dac to convert the digital information (usb) to analog if you want to use an rca input.
Does the avr have a usb port? If so check the manual for requirements. Probably need to format it with FAT32 as a mass storage device. A thumb drive might be the way to go, too (I use thumb drives on several avrs, I haven't tried an old hard drive on an avr.....I do use an old hard drive out of a laptop with a SATA to USB cable on a bluray player, but that wouldn't work in my avr as it has video files on it).