How can I Access my files from a different start up disk I am have problems with a Mac not starting up (macOS 10.13.6, MacBookPro 13” early 2015)  I created a new start-up disk on an external hard drive and wanted to recover files before going further but the Files on my user account on my laptop are locked - even though I am using the same Apple ID, name and password.
You'll need to add your user account to the folders and files using the Get Info (CMD + I). Unlock the gold lock and click the + add yourself with Read &amp; Write permissions to each folder and the files within.
Since that is quite tedious you can do it from the command line in an easier fashion.  Open Terminal and do the following:
($ is the prompt for a normal user and # is the prompt for root or sudo elevated equivalent. You don't type the $ or # symbols)
$ sudo -s (type your password when prompted)
# chown -R &lt;username&gt;:staff /Volumes/&lt;internal_disk_name&gt;/Users/&lt;username&gt;

It will take a while for the ownership to cascade down all the files in your home directory on the internal disk. Let it run.  The -R means recursively drill down the entire tree of files and folders from your old home folder the whole way.  There are likely a great number of files to process.
Once it finishes go back to Finder and you should be able to copy your files to back them up.  Then you can delete everything on the internal disk and re-install the operating system and restore your data from the external disk.