Backing up another user’s mid-2013 Macbook Air I have pretty much been a Windows user all my life but a family member has recently given me his mid-2013 i7 Macbook Air. This machine is logged in with his Apple account (I have the credentials) and I would like to reset it and start using it as my own machine with my own Apple account.

However, I would like to backup the machine before resetting in case he needs some files from it (he should have retrieved the files already but I’d like to be safe). My concerns are as follows.  


He has moved back to the Windows ecosystem and no longer possesses a Mac.  
Once I reset the machine with my own Apple account, I don’t know if I will have issue accessing the backup as it might be attached to his Apple account.  
I prefer a non-cloud solution, for example, a disk image that can be amounted (ideally from a Windows machine; please see (1)).  
If I back up the machine (storage capacity 128 GB) to an external disk with larger capacity, I would like that the unoccupied space of the disk is usable (ideally both with Windows and Mac).


How do I best proceed please? Please be detailed if you can as this will be my first Mac exp; however, please assume an otherwise generally competent tech user.
As the end result is to be able to access any forgotten files from a Windows computer in future, rather than to be able to restore to a new Mac, I would forget trying to backup the drive in its current form. 

The OS &amp; apps themselves would be of no use; &amp; could easily be re-acquired if ever needed, as they will belong to that user's Apple ID. It would also be reasonably safe to assume that any 'hidden' files wouldn't be portable to Windows - unless the machine was previously used for programming/coding. [Let us know if that's the case.]

That leaves you with, essentially, "Documents".
These would to a large extent be cross-platform, in &amp; of themselves - txt, rtf, jpg, mp3, etc etc
I'd say in that case that simply copying the entire User folder [Macintosh HD/Users/username/] would be more than enough. If you copy it to a drive formatted as ExFAT, it can live there as just another folder, accessible from any OS. You will probably be saving 'too much' data that way, but it will save having to make individual decisions on the contents of each sub-folder.

This solution does, of course, ignore any security considerations. The files will be in plain sight on the new drive.