Hello, did you safely eject it before disconnecting the USB cable? Also avoid USB extension cables or USB hubs. Also open the disk utility to see if it’s recognized there.
Help! This happned to us too. It has been working for over a year, and suddently our Mac won’t recognize it. Tried turning it off and on, unplugging, changing USB ports etc. Disk Utility does not recognize it either.
I am having the same problem with a Mac Book Pro 13 retinna display. USB3 port on left side will not mount the drive. USB on the right works. Drives are ejected properly. I have an open support request with Apple…something odd is happening. ther devices appear to work on left side USB3 port.