Formating a Western Digital hard drive

This generally means that the volume is formatted as NTFS, in which case it either needs to be reformatted or software such as Paragon NTFS installed onto the Mac. To check, select the volume in the Finder, choose Get Info, and look at the format.

In the event that this occurs for a FAT32, exFAT, or Mac OS Extended volume, either the volume's directory structure is damaged, or it's locked in hardware, or the drive has sensed that it's about to fail.

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If you don't need to use it with a PC, choose Utilities from the Finder's Go menu, open the Disk Utility, and reformat it as Mac OS Extended (Journaled).

If you do, proceed as above but choose MS-DOS or exFAT as the format, or install a product such as Paragon NTFS.

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If you intend to store and use you Photos Library from that drive it CAN NOT be any format other than OS X Extended (journaled). That's the only format that is compatible with a Photos library.