It means that the mirror (for whatever reason) is inconsistent. It is now rebuilding so they are both identical. With intel fakeraid sometimes even an abnormal shutdown can trigger a rebuild. Once it is complete, check SMART on both drives and see if there are any uncorrectable errors or media errors. If all looks clear, wait and see if it happens again. As always, the obligatory RAID is not a backup statement and make sure you have current, tested backups of everything that is important.
Yes. here are the steps. As always, before you make any significant changes to your array(s), make sure you have a complete and current backup of your data. Things can and do go wrong so a backup is always a good thing to have (especially since you now only have a single copy instead of a mirrored copy.
After looking in to it some more I canceled my drive order, apparently it was a "Fulfilled by Amazon" unit and has no real warranty. That got me looking into various other drives and it seems like most reviews are filled with great experiences and units dying after a month. A 2tb WD black which seems to have the least number of bad reviews is $120. Currently I'm only using about 700gbs and I can probably get rid of some of that anyway. I've just rolled over data from my last 3 machines because I have the space for it anyway
I went with a RAID 1 to alleviate worries of losing data but I wonder if I should just buy another 1TB SSD like a MX500 and call it a day. That would give me 2tb total SSD storage. I can then just back up the important files to my WD Blue like I do now.