I have seen this question posted a few times now, with people pretty much saying sorry can't be done.. Well I beg to differ 
1st. we need to get plug the external drive into you system.. After it comes up open a term and type this
sudo blkid
reviewing the output make sure to find your external drive and make note of its uuid.. (its a long string on alphanumeric characters, mine looks like this - C62A7EBA2A7EA6D9)
now that we have that we need to open the fstab and make some changes.. in the term type this
sudo cp /etc/fstab /etc/fstab.bak sudo gedit /etc/fstab
at the bottom type type this
UUID={your-UUID} /media/Media1 ntfs-3g fmask=000,dmask=000,uid=1000,gid=1000 0 0make sure to insert your UUID so for example on the one listed above (which I gave an example for) it would look like this
UUID=C62A7EBA2A7EA6D9 /media/Media1 ntfs-3g fmask=000,dmask=000,uid=1000,gid=1000 0 0
Basically we have told the system to mount your external drive to /media/Media1 (which is where you will find your find you files) with write access to group id 1000 (your group)
save the file exit and reboot your machine... Now you should be able to access your file on the external drive formated in NTFS on from within Plex at /media/Media1... Enjoy
