Files are missing. I was moving files on to new computer

This is exactly what happened. I tried to do this solution . . .
. . . but found it a bit beyond my knowledge. Im not sure about administrators and users. Im the only one on my computer so I dont know why there has to be an administrator.

I did get to the section where you click on ‘allow’ the person to view files but it didnt work.

I moved my files onto a WD external hard drive (product number: WDBAAU0020HBK-01) - Im assuming the 0 is zero and not the letter o.

I moved files onto the drive and then connected it to my new computer HP Pavillion 570 p059 (

When I connected the hard drive back to my old computer, only a few files can be seen.

I believe it has to do with permissions, etc as explained in this article but I was able to fix the problem.

Can you please help me

Thank you

I will add that maybe when I connected the drive to the new computer, it didnt recognize the user name because it is different
old computer: username is M1000
new computer: username is M1thou (the full name is M1thousand but those letters show)

I didnt purposely use a new username. I think / guess it was connected to the microsoft account which is

I dont know (shrug)