NUC Storage-Powered USB HD vs Non Powered

Just wondering what is better and why, running a 7i5 NUC using USB powered external HD or the non powered HDs for storage of 2TB of FLACS?

All hard disks are powered…maybe you mean usb powered 2.5” or externally powered like 3.5” drives? Or perhaps you mean the internally installed 2.5” inside the nuc?

All external USB HD.
Some larger physical ones have power supplies.
Some smaller physical ones are powered by a USB supply

You will find that most USB powered ones seldom support an external supply. Personally I use a NAS

2.5" disks use less power, so they happily run from the 5v power USB provides. 3.5" disks need more power so they must have an external power supply.

If your budget can stretch to a 4TB SSD, get one of them and chuck it inside the NUC. External HDD disks inevitably get knocked around and fail prematurely.