I bought the AK JR a few days ago and I have mixed feelings. The 10.000 Songs limit is a bit frustrating, but can be overlooked when you use the folder view - BUT... I have a big problem with a 256GB card from Samsung. Every time I reboot or start the device from a cold boot, the AK JR sees the 256GB Micro SD Card as 196mb - I always have to remove and insert it again. After a few attempts, it'll recognize the card and everything works fine. But when I insert the player into my computer, the system tells me that it has to format the external medium which is 196mb big.
Now if I use the card without the player, the computer recognizes the card as 256GB and everything works. When I insert a 128GB SanDisk card into the player, the player not only recognizes the card in a second, but also mounts it after a boot. And when I connect the player to the computer both cards (internal and external) show up in the device settings.
Has anyone tried another brand or has the same problems? I couldn't imagine that it is brand specific. And the website from Astell & Kern also says that a max of 320GB are possible (256 external + 64 internal)