This problem has returned today for me too. I was having this problem with a bunch of brand new 4TB drives Friday last week, but updated the plugin to 209.11.28 just after it was released, and it was all fixed. A few days later it, another update showed for the plugin, so I updated again, but noticed the version was still the same afterwards. Today I've thrown in an old but unused 3TB shucked WD blue, and it's failing: Invalid unRAID`s MBR signature.
I'm not sure if it really is failing, or if that problem has reared it's head again. Could that last update have just downgraded libevent? This is all far beyond my skillset.