Currently Being ModeratedOct 13, 2012 8:31 AM (in response to giruaro)
shut down your MacBook Air and disconnect the external drive from your computer.
Restart your computer into recovery mode (when hearing the startup sound, simultanously press both Command and R buttons). Once recovery mode has loaded, click on Utilities in the menu ribbon at the top of the screen and select Disk Utility from there.
Now reconnect your external drive and check whether it can be repaired from there.
Hope this helps,
Currently Being ModeratedOct 13, 2012 9:25 AM (in response to Christian A. Burkert)
Thanks for your message. I tried on recovery mode but it still says that I can't unmount the disk.
Doest that mean that it's really corrupted? Should I take it to a tech person?
Currently Being ModeratedOct 13, 2012 10:28 AM (in response to giruaro)
unless you are familiar with using command line tools such mount or fsck in a root shell while in single user mode, taking the disk to a technician would be advisable. At least, that is what I would do