I plugged a Firefly Smartdisk 20G hard drive which does not require external power into the edge. The drive spun up fine and there was a notification a new drive was plugged in. However, it was formatted ext2, and the edge reports the usb storage is either blank or using an unsupported filesystem.

I reformatted the USB hard drive as FAT32 on a PC and now the edge mounts it and shows files. It seems that ext format (a standard for linux which is what Android is built on) is intentionally not supported.

So, the problems with a USB hard drive on the edge would be the formatting, and how much power it requires.