Cisco ASA Error writing disk0:/.private/startup-config (I/O error)

The flash has got messed up. You can try:

• backup config  
• fsck disk0 
• If that fails format disk0
• tftp config back on

This is documented below but if you are under warranty you can just get a replacement ASA.

http://www.cisco.com/c/en/us/support/docs/security/asa-5500-x-series-next-generation-
firewalls/81884-asa5500-disk0-error.html