WD Passport external drive disconnects when iMac goes to sleep. Ideas how to fix? OS 10.13.5

Seems like a Mac problem not a WD problem.

Historically, WD Passport drives show up as troublesome around here a little too often for my tastes. The big issue is that they are "bus-powered" meaning they must get all power from the computer's USB port. With time and wear, the drive motor can start to demand more power than the port can deliver. WD bare drives are great but I've always found their enclosures less than satisfactory. They seem to be built to go on sale cheap at the Office Superstore every other weekend.

If you do not wish to abandon the Passport for a better device, there are three actions you can investigate:

• If you do not need to share the drive with a Windows computer AND you have a backup of the Passport, you can use Disk Utility to erase and reformat the drive. Set the partiton type to GUID Partition map and the format as Macintosh Extended Journalled. That sometimes helps the drive draw less power.
• Get a powered USB hub and place it between the Passport and the computer. These come with their own power brick and let the Passport get wall power that way.
• Get a "Y" USB cable (1.0 Meter (39") USB 2.0 A to USB 2.0 Mini B 5-Pin Cable - Auxiliary USB "Y" Power Design). It draws power from two USB ports, helping the Passport not feel it is starving.

I've used a number of "name-brand" external enclosure and every one has failed or acted bad enough to get binned. I now use only OWC enclosures with an independent power supply. I install the rather robust WD Black series drives inside. No more issues. This is the basic model I use on our iMacs:

OWC Mercury Elite Pro USB 3 with USB+1 Enclosure Kit at MacSales.com

You add what ever drive type/capacity you wish.

Don't put the computer to sleep, look at System Preferences - Energy Saver and then put Sleep to Never and then put the display to sleep. Much more reliable solution!