因修改usb驱动一部分文件，致mac book pro重启后，键盘，usb等外设全部失效，然后不停的google，重要找到方法了
在按启动的时候：按住command + R 就可恢复系统，
本以为会导致数据丢失，但是发现有重装系统的的选项，这样就安心多了。
http://support.apple.com/kb/ht1533
Keystroke | Description |
Press C during startup | Start up from a bootable CD, DVD, or USB thumb drive (such as OS X install media). |
Press D during startup | Start up in Apple Hardware Test (AHT). |
Press Option-Command-P-R until you hear startup sound a second time. | |
Press Option during startup | Start up in Startup Manager, where you can select an OS X volume or network volume to start from. |
Press Eject, F12, or hold the mouse or trackpad button | Ejects any removable media, such as an optical disc. |
Press N during startup | Attempt to start up from a compatible network server (NetBoot). |
Press T during startup | Start up in Target Disk Mode. |
Press Shift during startup | Start up in Safe Boot mode and temporarily disable login items. |
Press Command-V during startup | Start up in Verbose mode. |
Press Command-S during startup | Start up in Single-User mode. |
Press Option-N during startup | Start from a NetBoot server using the default boot image. |
Press Command-R during startup | Start from the OS X Recovery System1 |
