andre_6 its possible your disk:
-indeed had bad sectors
-format marked them as bad in the FAT structure
-ScanDisk scan skips FAT sectors market bad and only tests healthy ones, thus showed no errors
-zero filling the drive finally forced the Drive firmware to remap bad sectors, or drive already remapped them during the format. Modern drives have dedicated P-List/G-List/T-List tables for that purpose -are-p-lists-and-g-lists/
-now the logical disk surface is all healthy again