I sometimes encounter this too. I have an Amazon 5th generation Fire tablet HD8 with Fire OS version 5.0.4 with memory card 128 GB, This seems to have started after Amazon's last update of the Fire OS. This is a transient issue and doesn't appear to be the result of any program or system crash. I haven't removed the memory card since I first installed it last January. I also haven't completely powered down the tablet since the last Fire update either.