Hi! Just got an iMAC… I connect the usb external hard drive with my mkv movies, make it shared folder in “system preferences” menu, but cannot “see” it in “manage shares>add share option” in apple tv [appleTV flash(black) 4.0]. The “public folder.drop box” folder is located by the apple tv though,and when i copy a movie in it i can be reproduced just fine! Moreover the “PLEX” player can stream movies fluently via “plex media player” installed in the iMAC!
Just a few obvious things to check:
Under system preferences on your iMac in the sharing tab:
Is you external drive listed under the ‘Shared Folders’ column
Is the file sharing on tick-box enabled
Under ‘Options…’ have you ticked ‘Share files and folders using AFP’
Also, how is your external drive formatted? I don’t think this sharing works with NTFS formatted drives; you must use FAT32 or MacOS format. When I had this problem it was because when I moved from PC to iMac my ext drive was still NTFS. As far as I was aware, media player uses AFP and AFP does not work with certain drive formats. (I only ever got this working reliably when I formatted my external drive as MacOS)