I've never encounter any problem formatting drives and this is driving me nuts as I've recently moved to Sierra. Two brand new Seagate Expansion drives, they fail to format with disk utility. Not only that, they'll unmount and no longer are recognized.... like they are damaged or something. However the 'ghost' of the drives still show in DU and DU still 'sees' them. First thing I tried, I formatted to FAT and ExFAT and they formatted. But when I then tried Mac format, same thing happened. I'm thinking, maybe it's Sierra. I plug one of the drives into my old MacBook with Snow Leopard and the drives formatted. Hurray! Well no because when I connected to my Sierra OS Mac, the same bloody thing happened when I tried a format. Oh Yeah, and DU is telling me these drive have 2.2 or 3TB USED space! You might say well why didn't I leave well enough alone but even still, if Sierra won't format then something bigger is wrong. There is definitely something up with Sierra. Oddly enough I also recently bought a 6 & 8 TB Seagate's and had no problem formatting them! I can't recall but I may have formatted those earlier two before the last Sierra update. Hmmmm. Any ideas folks?