Was asking if the windows on each PC were the same version because sometimes a higher lever version of windows uplevels information on the HDD when it reads it, causing the lower level windows problems when it goes to use the disk. But since you are both win10 that should not be a problem.
It was unclear, but I was asking you to run the file system repair utility first and see if that brought back the files. That was the "right click the disk icon, properties, tools, error checking" step. That causes windows to read the files in the file system and cross check that everything is linked together properly. If the missing files still exist they can be found that way. You can also look in "recycle bin" to see if they are there.
The final line "virus scan" was for the case where 1. the files disappeared 2. no one erased them and 3. no disk errors were found. The question wasn't for your PC, the files did not disappear there, it was for your brother's PC.