(The ASA5520IOS recovery process (verified) failed to upgrade my 5520 a few days ago, causing the device to restart repeatedly and only enter the ROMMON mode. Then I looked for several recovery methods on the Internet, but they were all unsatisfactory and presented various problems. Through my own test, I will now post the successful recovery process that has passed the verification. I hope I can share it with you and wait for a very good method.)