Doug, the rear USB should have enough power in order to use a USB external hard drive. However, we have seen an issue where customers are not able to boot when the USB hard drive is connected to the NUC.
If you turn on the NUC without the USB drive, and once it boots to the OS, you can plug the hard drive and you will be able to use it without problems.
Some customers were able to fix this behavior by enabling Fast Boot on BIOS. If you do so, please keep the “USB optimization” disabled, in order to access the BIOS