Did it mounted successfully last time you plugged into the Mac. Here is what Western Digital recommends you to do when WD doesn't show up How do I use my WD Passport external hard drive?
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Like our subreddit? Want to pay it forward? Consider donating to the Electronic Frontier Foundation! They give you stickers! I have this external hard drive (WD My Passport 0730) which worked just fine a couple hours ago. 
But now it won't show up on any of my computers.
When I plug it in, the drive itself does light up and it sounds like it's working though. On Mac, nothing happens at all.
It's not in Finder or Disk Utility. On Windows, it makes the USB plug-in sound, and says 'Installing drivers' or whatever.
When I open the thing trying to find drivers, it's searching for 'WD SES Device USB Device' and 'WD My Passport 0730'. The SES driver says it can't be found, while the 0730 driver just says 'Failed' and changes to 'disk drive' instead of the model name. It doesn't show in Disk Management, and in Control Panel it just shows as 'WD SES Device USB Device' with a yellow exclamation. I've tried it on Windows 7 Pro 64bit (desktop PC), Mac OSX Yosemite (Macbook Pro), Windows 7 Pro 64bit (Macbook Pro with Bootcamp). I've also tried two different cords, both of them work with my other WD hard drive on the same systems. I also tried the WD Data Lifeguard Diagnostic program, but it didn't show up in it. Hello mintorment!
This is a bit frustrating.:/ How old is the drive? Does it have data on it that is not backed up and you need it back? If you can hear it spin, then it's being powered, at least, but the data connection seems to be not working. The click you are hearing doesn't sound good, it could be the drive is failing, or this has already happened. I see you have tried the drive on both Windows and Mac, what is its file system? Unless its exFAT or FAT32, it would be working with read/write processes on one of the two systems, only. Change the USB cable again with a new one, shorter than this one, and depending on the file system connect it to the PC or Mac.
See if it connects then. Connect it to the PC with the new cable and open diskpart: • Open the Start Menu, type diskpart, press Enter • Type list disk, press Enter Is the drive there? Is it in Device Manager? If it is in Device Manager, try to manually update the drivers from there. If it doesn't work, Uninstall the drive from there, reconnect it, let it automatically reinstall and check again.