You have several issues:
The Seagate volume is an NTFS volume (used for Windows systems usually) -- you need special software to check/mount those types of volumes, e.g. Paragon or others. If you have this software then the mount command would need to have "-t ntfs" instead of "-t hfs" I think (check with your software provider).
You need a space between the "/dev/disk1s2" and the /Volumes/Seagate... and you also need to enclose the /Volumes/Seagate... in quotes or escape the space character in the volume name -- should be something like "/dev/disk2/Volumes/Seagate Backup Plus Drive" and you also need to make the folder name in "mkdir" command using /Volumes match. Are you certain it's the full name "Seagate Backup Plus Drive" and not just "Seagate"?
What software are you using to backup to this NTFS volume? Did the "Seagate" drive come with software so that you could use it on Mac's?? Unless you want to use this drive on another Windows system you might want to think about formatting it as HFS+ (since that is native to Mac systems but Windows systems would have a problem reading it without third party software) but realize you will loose everything on it when you do this. Is there anything on the drive at all you wish to preserve?
Also please do this in a Terminal window before doing anything else:
diskutil list
and also do
df -kt
so we can see some important information. Also keep asking questions before doing anything that cannot be undone so you understand the issues.
Good luck...
