FYI here is one report of a similar issue - I don't think my card unmounted though, it was just not writeable to by the amazon video app, yet OK with the netflix app...
https://www.amazonforum.com/forums/d...deos-on-new-hd
for me - having an SD card is all about reliability - if I can't trust to to always work, & to always play back what is saved to it,. then no sense having one...