I hope this never happens to you, but in your illustrative Mac career one of your hard drives will fail. Yes, even an SSD drive can fail or more accurately they just use up all of their available cycles. There are signs that a drive is failing, this article describes several of them. The article also goes through several standard tech support procedures for a failing drive on an older MacBook Air, they would be good to know. If or when one of your drives acts like the one in the above article, it may be time to replace it. You can have Apple or a third party do the replacing, or you may wish to try it yourself. I have replaced several SSD Flash Drives in Airs over the years. The solution by the author of the article is doable.