On restarting the Mac, the problem had disappeared but reappeared later when the computer woke up from sleep. A techsupport person at Apple suggested reformatting the drive, but Disk Utility could not unmount it.
Then although its icon appeared on the desktop and its contents were visible, I got error -36 when I tried to save/copy files onto it.
I think I have solved a similar problem which appeared intermittently on a new Mac Mini, running Mavericks OS.
