Some of the newer Android devices (like the Nexus 7) don't use USB mass storage, but rather use a different protocol called MTP (Media Transfer Protocol.) It's a bit harder to deal with than mass storage -- you have to use a dedicated application instead of just mounting it as a drive -- but it has the advantage of not taking the drive offline from the device itself during the transfer.
I was a little irritated when I first ran up against it, since I was unaware of its existence and didn't have the appropriate software on my computer, but once I installed it I was fine. It's a bit more work, but not really very complicated.
It seems to treat the drive as a network resource rather than as a drive. They made the change so that you didn't have to have separate partitions for media and apps -- manufacturers were including huge storage areas for media (as USB mass storage) and people were running out of room for apps (which needs to be on internal storage), even though they had lots of free space. Making it all one partition and then using MTP to transfer files solves this.
I'm not aware of any ereader that uses it. No reason it wouldn't work, but an eink reader doesn't have the same need that a tablet does for flexible storage. That means the manufacturers are less likely to take this step.