Data recovery from 2012 MacBook Air SSD using 2011 MacBook Air
Added as an answer after comments...

Initially a guess, that the disk was formatted APFS, which would be unreadable by El Capitan.

Upgrading to High Sierra would enable the disk to be read &amp; written as normal.

As regards permissions; you could either add yourself to the user list, as an ACL, or you could just Get Info on the drive itself &amp; check the 'Ignore ownership on this volume' box.