1 File found but 3500 ignored - why is that and can ignored files be recovered ?

A friend asked me to look at an external harddrive and recover missing pics, apparently they loaded the drive to mac computer an formatted it in error. I ran a deep scan and selected the option to scan for non deleted files( for reformatted drives)

It ran for nearly 10 hours and was counting files as it went, however at the end of the process it found 1 file for recovery but had in parenthesis that it ignored over 3500 files - why is that and can the ignored files be recovered ?

Ignored files are usually hidden system file, zero length files, and non-deleted files. However I have no idea what formatting a drive on a Mac entails, and what state the drive is in after the format, unless it's FAT32. Recuva needs a Windows disk format to run on.