Ok,
First of all I do not have nor have access to a device that fits in this class however I've been attempting to keep up on this topic. That being said, it appears that some of these devices are using storage controller drivers that refuse to use (or recognize) the auxiliary SD card slot as a viable secondary drive (to view your storage controller(s) (there are typically more than one) open Device Manager and expand the Storage controller section). In some such cases using the Windows Disk Management tool to assign a permanent drive letter (recommended anywhere from G thru Z) can force the change to recognize the drive as a secondary HDD, but this too is not always. One of the things that come into play with a Feature Update is that it is actually an upgrade type installation to a new version of the OS and therefore will/wants to create a Windows .Old folder for the purpose of being able to have the option available to the user to choose the Go Back to your Previous Build if the need or want be (the 2 together, for all practical purposes, is then the size of the OS, plus personal files, install x 2). In this is were the trick lies, in having the Windows.Old folder installed to the secondary drive instead of the primary drive. With devices were this becomes problematic, or just not possible, a backup of the personal files to the SD card (or other external drive) and then performing a truly clean install of the "Feature Update" from the downloaded ISO proves to be successful (unless of course there is a hardware/driver incompatibility underlying). Of course this is/can be a PITA as all non default apps and programs would then need to be reinstalled but, then again, unless the device was an early on model of a tablet or such and there is no upgrade possible, the device manufacturer would love to sell you an upgrade to a 64GB, 128GB or whatever upgraded storage capacity. (This was probably even given or listed as choice when the device was initially purchased).
I truly wish that I had a better answer for you here other than the last resort of a fresh clean install anew (however, a truly clean install is the best by far (much better than a factory install). For quite a bit more info accordingly you can check out the link below:
Good luck and do let us know how it goes.
-Richard