I purchased a WD My passport for Mac £TB yesterday. I have tried to back-up my iMac three times and each time the Mac has shut down with the ? folder…
Is this a common issue?
Hello Stephen_Bain! 
What software did you use to create the back up? Does the iMac shut down with the WD external connected while performing other actions besides the back up process? Please post a screenshot of the it/the error you are seeing, including the folder in question.
I would suggest you the most simple steps first: use another USB cable, use another port/a hub, test the back up process with another external, use the dedicated software to check the drive: WD Drive Utilities for Mac (directly downloads the software), do the short test and also check the SMART status of the drive and post screenshots of the results.
Please check your system statistics and temperatures when trying to create the back up. The system could be overheating. Also, have checked the WD external if the back up hasn’t after all completed?
Post back with the results!
CK_WD
The hard drive is also not recognised by my MacBook.
I think I have a faulty device…
Does it spin when powered on? If it by any chance shows up in Disk Utility, try to reformat it (this erases data). Remember to change the cable! If you can, you could also try on a Windows machine or a third OS X machine, just to make sure that it doesn’t show up at all. 
If other systems cannot see the device, even with a different cable, this most probably means an issue with it, so I would advice seeing to RMA it. Here you can check the external’s warranty status and this is the WD Warranty Policy.
Best of luck!
CK_WD
The disc works with a MacBook Pro - I am currently copying all of the files that I copied, back on to it - but messed up my iMac (giving the folder with a ‘?’ in it) and couldn’t be accessed from my MacBook.
I’m assuming this means the cable must be fine…
If you are using the same cable with which one time the drive is recognized and the next – it is not, it could still be failing. Test the connectivity of the drive with a new cable. If the external is still recognized, please run a health check on it using the WD Drive Utilities for Mac (directly downloads the software). Do the Quick test and view the SMART, post screenshots of all results here. I would too suggest you to test the back up process on your iMac with another drive, see if it works.
Good luck!
CK_WD
I’ll try this later
Thanks
