If you purchased a 64 GB or 128 GB MacBook Air (mid-2012) between June 2012 and June 2013, your flash storage drive might be defective, in which case Apple will replace it for free.
To determine if your MacBook Air has a faulty flash drive, open the App Store app on your MacBook Air, click Updates, and download the MacBook Air Flash Storage Firmware Update 1.1. Presumably, if the update doesn’t appear, your MacBook Air is not among those that might suffer from this problem. (TidBITS publisher Adam Engst has a mid-2012 MacBook Air, didn’t see the update in the App Store app, and when he downloaded it and ran it manually, the installer reported “This computer does not need this update.”)
Once downloaded, the update will test your drive and install new firmware to resolve the problem in most cases. If the drive is faulty, the update directs you to the relevant Apple support document for further instructions about how to get a free replacement. If the update completes without any additional comments about the drive’s health, we believe that means the firmware update has fixed the problem, based on Apple’s statement:
Apple has discovered that a small percentage of flash storage drives in these MacBook Air models have an issue that may result in data loss. This update tests your drive and, in the majority of cases, installs new firmware to resolve the issue.
If your drive is bad, Apple strongly recommends that you don’t install new applications or operating system updates until the drive has been replaced. You should also be sure to perform a backup as soon as possible. In fact, you should do that even if your drive is good! If you’re not backing up regularly, be sure to check out Joe Kissell’s “Take Control of Backing Up Your Mac, Second Edition,” which tells you everything you need to get started.