a friend had her 2013 MBA SSD replaced by a larger SSD from OWC. This happened just last week. The work was done by a Mac specialist. It worked ok for a week, then suddenly it would not open up. The OS progress bar got so far,then it shut down the MBA. So I opened it in restore mode, checked the SSD for repair and it returned a huge number of directory errors. It will not repair and instructs me to erase, replace OS. Unfortunately the MBA is in France and the Mac specialist is in the UK. So probably it will have to wait until this friend gets home. But I'm a curious guy and I'm trying to workout what went wrong. Hardware issue. Incorrect installation of SSD. Seems unlikely though. The old OS had directory errors which weren't repaired before transfer and the transfer made it worse If the old SSD was clean and clear, I don't see how the OS install and migration would cause so many errors to happen and break the OS. I've done this many times and never seen anything quite like this. I also don't have access to disk warrior, so that tool is unavailable.