An associate at the Men's Shed today copied a Numbers doc onto a USB stick using his Mavrix MBP then found the drive would no longer mount on either his MBP or the Shed's Sierra iMac.
Disk Utility could see the drive but it wouldn't mount.
DiskWarrior to the rescue, the directory was too badly damaged to allow automatic repairs, so I chose to replace said directory with the new one that DiskWarrior generated.
Easy fix, the drive mounts again & is fully usable.