USB connected HD,which format?

Running rock on a Nuc 7i5 with 2 connected USB drives for FLACS.(no windows/Mac)
What format should the 2 connected USB HD be?
I use Mac for everything else

Searching this very forum yields

So if I move my music from the OSX formatted HD to my new 4TB ExFAT formatted drive to use with the NUC.By doing this on the Mac will have no detrimental effect on Roon running from the ExFAT drive ?

Should be fine, and faster than copying over the network. Connect the USB drive first to the Mac to copy across your music to the drive; then connect the drive to the NUC and use in Roon.