I have only had one problem with my lovely new E-M1 II, and that has been the occasional unreadable file, or a file with colours smeared across it. Only happened from the high speed UHC-II card slot, and only happened occasionally. I was not the only one to have this problem and we hoped for a spermware* fix from Olympus.
*For new readers, my wife once referred to firmware as spermware, a slip too good not to use for ever.
I first had the problem on a hot afternoon at Jim Thompson’s farm; and then no problems at all for many weeks, such that I pretty much forgot about it. Then, on the first day of a polo event, I stood in the very hot sun for a couple of hours and when I returned home I had about 5% failed images; not good at all. So the next day I wrote JPEG files to the second card as well as RAW to the fast card. I also turned off the camera when the pack went to the other end of the field and didn’t cradle it in my hands all the time. Result: 100% clean images, so maybe heat exacerbates the problem.
Still, with another event the following week, something had to be done. I was using a Sandisk 64GB which apparently gives more problems than the 32GB version; so at some expense I ordered 2 x 32GB cards from AVcamera. One hour after a placed the order, Olympus duly released a firmware update that purports to solve the problem. Well done Olympus, but if only you had announced it a couple of hours earlier….
I now have more UHC-II cards than I need…