It probably couldn't see them because the formats were something other than what the PS3 can use. The PS3 can see JPEG (DCF 2.0/Exif 2.21 compliant), TIFF, BMP, GIF and PNG images.
For music files it can use ATRAC, MP3, MP4 and WAVE (Linear PCM). If the music files have copy protection on them they may not work on the PS3.
As for videos they must be in MPEG-4, MPEG-2 or MPEG-1 format. Files such as Quicktime, .wmv or .avi would not work.