My Seagate USB backup drive has 6 folders in one partition formatted NTFS. I have the USB Backup plugin installed. The drive is recognized, and I can mount it easily.
2 things: the drive does not show in the plugin "device" list; and it is not 'referenced' in the file systems page but is online.
OMV6, updated and ready to move to 7, on a Lenovo tiny connected to my home LAN.