If you have upgraded your main drive with an Intel X24 SSD drive, your Mac may fail to boot after updating to OS X 10.8.3. Symptoms include hanging during startup, or a circle with a slash being displayed. The fix is to update the firmware on the Intel X24 SSD drive, which requires using a different computer to download an ISO image from Intel containing the latest X25 SSD firmware, burn this ISO to disc, then boot from the disc to apply. Depending on your Mac setup, this may require using an external SuperDrive if you don’t have the required internal SuperDrive.