Please check my post on the other thread.
http://forums.sandisk.com/t5/SanDisk-iXpand-USB-3-0/iPhone-X-with-11-1-2/td-p/364188/page/2
I believe it’s a firmware issue with the iXpand drive, because I have one that works perfectly fine on both iPhone X and 8 Plus running the latest iOS 11.1.2. The working iXpand drive came with firmware version 2.60.115.88, while the other two that didn’t work came with version 2.60.103.88.
