David, I’ve noticed the same phenomenon a few times too.
About five weeks ago, I bought a Western Digital 4TB hard drive for my itunes files (connected to an iMac). Worked without a problem the first two weeks, then was constantly ejected without any noticeable reason. I had unchecked the option to put hard disks to sleep when possible, but saw no difference. The ejecting continued for about 10 days and now, without any further adjustments, the HD has stayed mounted for over a week! I’m keeping my fingers crossed and hoping that the strange ejecting is over.
It could be a bug with a recent (a few days ago) Apple update to itunes and the OS.