Upgraded to 10.10.5 and now shutting down when accessing USB external drive

I'm a graphic designer using a MacBook Pro plugged into a 27" Thunderbolt monitor with Adobe CS6 software installed on my laptop. All my data is stored on WD external drives. Until I upgraded to 10.10.5 I never had a problem. Now every time I am accessing data from the WD drive the screen locks up, the spinning wheel appears on my laptop screen and the computer just totally shuts off. No warning, no error message…just literally shuts off.

When I reboot everything I had open reopens exactly to where I was at. All windows open. So I save my work and start closing programs. Without fail, Suitcase Fusion 6, Photoshop, InDesign and Firefox will say "Force Quit". I close all windows, shut down, wait a few minutes and restart. Reopen my software and proceed to work.

There is no rhyme or reason, or consistent length of time before my laptop shuts off again.

I have had my MAC expert come to my home (I freelance and work from home) and we have been able to recreate the problem. After thoroughly checking out all my hardware, software and laptop his conclusion was that it's a bug in the 10.10.5 upgrade that is conflicting with the USB connection. His suggestion was to copy the folder I am working with to the desktop and then put it back on the external drive when I am finished. This just isn't possible as I need to continuously access the external drive for graphic images, text, etc.

Has anyone else experienced this problem? If so, were you able to fix it?

Both Apple and Adobe suggested I upgrade to High Sierra. High Sierra isn't compatible with CS6 so I would have to subscribe to Adobe's CC software at $50.00 a month. Also, my laptop is an early 2013 and I'm not crazy about putting High Sierra on it which would mean a 'new' laptop.

Dang. But this is a miserable mess.