Apologies for the late reply, work got in the way again
Great deal of information you have all provided and I thank you for the time and your efforts.
It is appreciated knowledge shared with others, who undoubtedly will come across these areas when experimenting with lightning and HDD's.
Here what I have gleamed in the past few days, re lightning and iPad Pro's.
One video I watched the presenter actually had the same Sand disk 2019 USB pen drive as I and also the WD elements HD.
In his video the USB did not work, the WD flickered on and off.
Ironically the older USB pen drives worked for him, exactly the same for myself.
His conclusion is the same as you mentioned Tony.
"flogging a dead horse with HDDs."
So for others reading iPad Pro with USB 3 speed with lightning port, simply put it is un stable when using HHD's. (Except HHD's in a Stardom self powered set up - this unit is for hot swap older drives and repeatedly worked without issues, when using a iPad Pro 1st gen and camera powered adapter)
Other newer models iPhones, will not support HDD's.
Here is what I have found does repeatedly works and is stable with iPad Pro 1st gen lightning port.
I have included some basic transfer speeds for others.
1.57 GB and a 534.5 MB (that is 2 files)
iPad Pro 1st gen lightning port to SD card - 04.47.35 minutes.
SD card to iPad Pro 1st gen lightning port - 00:29.55 seconds.
From experimenting with all of these different transfer methods, for me SD card works better.
1: No power required.
2: Can move about freely, not tied to a table with a power lead.
3: Can carry my Stock video and Stock Raw images easily with the tablet.
Only negative here is the iPad's write out speed is capped heavily, so moving files off the iPad still remains a slow process.
So I have moved some of my Stock image library over to the SD card using a laptop.
225.72 Gig's 2 hrs.
Using files app with the SD card also removed the issues with WD app that will not give previews on some Raf files and Png PSD etc. (WD my cloud)
In fact working from the SD card visually, is just like being on a desktop finder scrolling through the images with ease and no lag in previews. ( Average image size 25 meg's and above)