Since a system upgrade my macbook fails to boot properly: a loading bar appears, it takes about 15 minutes to get filled and then the system shuts down again.
When booting in recovery mode, I tried to "Repair" the main partition in the disk utility tool but an error message told me there actually was an issue but it could not be fixed and I needed to format/reinstall the system.
I also tried to erase the bootable partition but it also failed (saying the drive could not be unmounted).
I tried to choose "Reinstall Mac OS X" from the main menu, but my main partition was not found.
I don't have any bootable disk or USB drive of Mac OS X. Is it possible to create one from a windows PC?
What else could I try to fix the problem?