For some reason, I have to erase and format all my flash-drives twice, because it fails the first time. It happens on all USB ports in back of Mac-Mini and with different Lexar and Verbatim USB flash-drives. Seems to always fail the first time, then completes the second time. They format on my other Windows machine fine. I don't recall this happening before I upgraded from El Capitan to Sierra a week or so ago. At first, I thought maybe my USB ports were going bad all of a sudden. However, the ports work fine for other things. I found these in Google: https://apple.stackexchange.com/que...ternal-disk-twice-first-time-will-always-fail and https://discussions.apple.com/thread/7816322?start=15&tstart=0 I guess it's good to see I'm not the only one having a problem with it. At least I know my MacMini is likely not broken. I'm guessing someone has officially reported it, but interesting to see it's still broken on v10.12.6 . Maybe it's only happening on certain machines? Is anyone else here having this problem? Is there a known work-around?