I'm trying to format my USB so I can get all the space back onto it, but every time I try to erase it in Disk Utility, this comes up:
POSIX reports: The operation couldn’t be completed. Permission denied
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Although it may seem unintuitive, try formatting the drive using a Windows computer. Then, take the now (Windows) formatted drive over to the Mac and format it there - it will work.
It's has happened to me many times too, but the speed of this method has always greatly outweighed any further research time.
I hope that helps you out, however, the question of what causes this error could be one of many... hence, if you need a formatted drive now, this will work.
But do let me know how you get on.