Welcome to Tom's Hardware, stanleigh!
I'm sorry to hear about your connectivity issues with the WD My Passport, but I'm afraid that the tablet (and its microUSB port) cannot supply the external with enough power even with a SimulCharge USB. You could try connecting the WD external while you have the tablet charging. Does the drive spin up anyhow?
However, unless the tablet can supply 900mA through the USB port, you won't be able to get it powered up. You can refer to this thread from the WD community:
http://community.wd.com/t5/WD-Portable-Drives/Samsung-Galaxy-TAB-S-amp-WD-My-Passport-Ultra/td-p/813853
Good luck!
SuperSoph_WD