Chosen Solution

I bought a MacBook Air A1369 with OS-X Yosemite, I started update but after rebooting the system was blocked on the charging bar, I let it for for more than 8 hours and no result!!! I retried to reinstall Yosemite but same result again, the fan is working non stop, I tried to boot up the MacBook Air using my MacBook Pro’s internal HD as external drive (Target Disk Mode), it boots but it takes a long time to boot or to start an app.

You might want to the onboard diagnostics to check things out. Hold the D key when you boot up the MacBook Air. You could do the reverse! Boot up your MacBook Air in Target Disk Mode then use your MacBook Pro to reformat the Air’s SSD drive using Disk Utility as well as install a fresh OS on it from your MacBook. The Trick is to make sure you select the Air’s drive. You may want to rename the Airs drive before you do the install so you don’t get confused if you have the default name Apple uses on your systems. To speed things up I would interconnect the systems using a Thunderbolt cable.