Installing to an external drive
The macOS Recovery system can install the OS to an external drive, then boot from it. If you use an SSD, you may find the speed of your Mac improves considerably, as these drives have no moving parts and use ‘flash’ memory. Your best option is to replace your Mac’s hard drive with an SSD, but this can be tricky compared to plugging in an external device, and could invalidate your warranty. To proceed, connect the external drive and follow the steps detailed in this section, but choose your SSD (under External in Disk Utility’s left pane) instead of the Mac’s internal hard drive. Select the SSD itself, not a volume on it. Once complete, hold down å as the Mac starts up and select the SSD to start up from it.