I initially had some problems backing up from my 256GB iPad Pro to the 128GB iXpand drive and after some correspondence with Sandisk they discovered there was a problem with using a 128GB iXpand drive with the 256GB iPad Pro but they had tested using a 256GB iXpand drive and it worked fine:
Thank you for your patience. We discussed this case with our L3 technicians and found out that we were able to recreate the issue with the 128GB iXpand Drive whereas with the 256GB iXpand Drive there were no issues. Also, we are looking forward to resolve the issue with the iXpand USB 3.0 128GB drive in our future updates.
Further, in this case, I would like to offer you a replacement for your product with the iXpand USB 3.0 256GB Drive
I got around this problem on my trip by loading from my camera to iPhone 7 to iXpand drive and it worked fine.
When my replacement 256GB iXpand drive arrives I'll test it and let you know how it goes.