Seagate has an external 8TB drive, the STDT8000100 or "Backup Plus" that's available for ~$300 on Amazon.
The Backup Plus uses Shingled Magnetic Recording (SMR) to achieve the required bit density. It works by overlapping magnetic domains kind of like roofing is overlapped when it's installed. It uses 6 1.33TB disks and is rated for about 20 full read/write cycles per year. It can do 124MBps sequential reads and 195MBps sequential writes(!)
It's not particularly good for heavy write/rewrite applications, but it works great for high burst speed writes and periodic high speed reads – making it ideal for backups. It's got a rather large cache (128MB), used by the SMR technology to achieve the fast read/write specs. It communicates with your computer via a micro-USB port.
So, how much is 8TB?
You could fit the contents of more than 150 Blu-ray disc on it. Or 1500 DVD's. Or six million 3.5" floppy disks worth. Or 8000 copies of the Encyclopedia Britannica.