Open Disk Utility, and select your drive. Does it say Mac OS Extended or FAT32? If it’s the latter, you’d do well to reformat it to Mac (bear in mind that this will erase everything on the drive).
And David:
“USB does not deliver the sustained throughput required for video editing, period, end of story”
That might not be a good phrase to throw around these days, with both LaCie and CalDigit putting out USB3 drives that most definitely ARE capable of video editing.