Yes - I experience the issue you describe every once in awhile. The first AK100 I had would display the error about every third time I turned it on.
IRiver acknowledged the problem and said they are working on the issue.
The new unit I have now has booted up and both 32G cards didn't show up maybe 3 times in the month I've had it. The remedy is to turn the unit off and then back on - the cards should show up and work normally.
I've also experienced the issue when listening to a long passage (longer than say 25 minutes) - usually a classical piece I'm listening to. The unit will freeze for a few seconds and then I'll get a message telling me the song I've selected isn't available and both cards disappear. That's happened once.
I'm hoping they fix the issue soon.
I have only experienced the not detecting the sd card is when I removed the sd card from the device and reinserted it back in.
I have never experienced any hanging or loss of detection while the AK100 is in the midst of operation or when turning it on before though.