I have a WD My Passport Ultra and I had it plugged into my MacBook Pro 15" Mid 2012 Model and I had just closed the lid and it went to sleep. Then I came back hours later and when I opened it a message was stating that it had been improperly ejected even though it was still plugged in. I wanted to know if this was normal and if anyone else is experiencing this.
It happens occasionally with my external drive which I suspect is a connection problem. I run 10.7.5 so I would judge it unlikely to be a Yosemite issue. If slightly moving the cable around replicated the behaviour, that's probably it.
But no, clearly this is not normal. It would not hurt to try it on other systems which I don't currently have the hardware to do.
MacBook Pro (15-inch, Late 2011)
Every USB hard disk, flash drive, or SD Card I have ever connected to this computer occasionally reports this error when the computer wakes up. It doesn't matter whether the computer went to sleep after non-use, closing the lid, or low battery hibernation. The only workable solutions I know of are to disable sleep, never drain the batter completely, and always disconnect every external disk immediately after use.
This is not a small problem. Disconnecting a device without first flushing the write cache poses a serious risk of data loss and/or filesystem corruption.