How to Get Your Data Off a Mac That Won't Boot
How to Get Your Data Off a Mac That Won’t Boot Table of Contents Got a Recent Backup? Use That Best practice dictates that you regularly back up your Mac to an external location. You can use Time Machine and a simple external USB hard drive to do this or set up a more elaborate networked solution. When the worst happens and your Mac fails to boot, you can simply plug your Time Machine disk into another Mac and access your files from there instead. This method assumes you have a recent backup and that the backup includes the files you wish to access.
Source: howtogeek.com