I am having the exact same issue. Here's my story. I started with two drives, both of which were originally ntfs drives that were once successfully mounted and sharing files not only across the network, but to my xbox as well, but only after much tweaking.
Currently, both drives are formatted using a GUID partition table, ext4, and have a only 1 partition each. Each drive has its own entry in /etc/fstab as follows:
I manually created two files in /media as follows:
# Storage 250GB /dev/sdb1
UUID=9ef74a0e-ac8e-4cd0-bc0f-96869dd5376c /media/Storage ext4 defaults,user,noexec,nosuid,rw 0 0
# Media 200GB /dev/sdc1
UUID=63231c7d-5f6b-4ee9-8e7b-b4769e0b9408 /media/Media ext4 defaults,user,noexec,nosuid,rw 0 0
Each drive is listed twice in nautilus and can be mounted and unmounted by me. Also, I cannot share any folders on the drive. When I attempt to share the folders through nautilus, i get the following error:
:~$ sudo mkdir /media/Media
sudo mkdir /media/Storage
sudo chown zac -R /media/Media
sudo chown zac -R /media/Storage
'net usershare' returned error 255: net usershare add: failed to add share music. Error was Operation not permitted
I think this problem has something to do with ubuntu automatically creating folders in the /media directory when there appears to be a hot-pluggable drive present, but it could also be something as simple as changing a setting in nautilus. Either way, any help would be greatly appreciated.