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 & 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 & check the 'Ignore ownership on this volume' box.
