Yes, you can play the contents on a USB hard drive. There are a couple of different ways to do this.
If you do not need to see metadata tags like album cover art, you can just connect a USB HDD or Flash Drive to one of the USB data ports on the back of the Aurender. You can then use the Folder menu option to play files from the USB device.
If you would like to see the metadata tags of the music files on an attached USB drive, you can run AMM while the drive is attached to the Mac/PC. After AMM finishes, you can detach the USB drive from the MAC/PC and attach it to the Aurender. You will see all the metadata tags from the drive as if it were an extension of the internal hard drive of the Aurender.
*Note that we have found a few DACs are not compatible with Aurender when used with an external USB device (playing from internal storage or a NAS drive does work). The following devices have known issues when music is played from a USB drive- Ayre QB-9 USB DAC, Grace Design m903, Berkeley Alpha USB(USB to SPDIF converter).
*Also note that Aurender music servers can only address USB drives with a single partition. If the drive has multiple partitions, then it will not work and will need to be formatted for one partition.