What a pain! PITA!
Got there in the end. This was me trying to flash a Raspberry Pi Lite image to a Samsung Pro Plus Micro SD card via an Orico USB to SD card adaptor. Brand new card and brand new adaptor as my old one was reading incorrect sizes on old SD cards.
Step 1. Disable Protected folder access in Windows Security settings. It was not enough for me to add a program-specific exception to the list. (Remember to turn this back on when finished!)

Step 2. If you have a half-baked failed etch written to your SD card, open up disk manager (start\run "diskmgmt.msc" and delete any partitions that are there -
This might have been the crucial step?Step 3. Reboot your machine. Not sure if this helped or not but my machine locked up completely and after I powered it down and back on again (and followed the previous 2 steps) I got a working etch so who knows!?
Step 4. Change your USB port to a decent one. I was using one on the top of my case. Not sure if that one was 1,2 or 3 but I didnt trust it so I used a blue USB 2.0 port on the back of the case, directly attached to the Motherboard, and this might have also helped. (this was actually what caused my PC to lock up and need a reboot in step 3 above... Worried I might have some dicky hardware now!?!)
Step 5.
Turn Windows Protected Folder access back to "on". You need this! I need this. Everyone needs this!