If your 8TB extrnam HDD is made up of USB Hub and disk, which means it contains 2 or more small disks, so the archer C1200 will not detect it.
Maybe you can contact TPlink support for the further help.

http://www.tp-link.in/faq-603.html

[FONT=AktivGrotesk-Bold]2. Capacity
[/FONT]
Supports at most eight partitions in theory, if the hard drive has more than eight partitions, it just load the first 8 partitions. (On the same drive, the total capacity of each partition cannot exceed 2TB on WR842、WR1043、WDR3500、WDR3600、WDR4300; If there are different hard drives connected via an USB hub, in theory it can identify a maximum capacity of[FONT=AktivGrotesk-Bold] 8 * 2TB.Similarly, the other models with USB function like archer routers, the total capacity of each partition of usb drive cannot exceed 5TB )[/FONT]

http://www.tp-link.in/faq-603.html


[FONT=AktivGrotesk-Bold]2. Capacity
[/FONT]
Supports at most eight partitions in theory, if the hard drive has more than eight partitions, it just load the first 8 partitions. (On the same drive, the total capacity of each partition cannot exceed 2TB on WR842、WR1043、WDR3500、WDR3600、WDR4300; If there are different hard drives connected via an USB hub, in theory it can identify a maximum capacity of[FONT=AktivGrotesk-Bold] 8 * 2TB.Similarly, the other models with USB function like archer routers, the total capacity of each partition of usb drive cannot exceed 5TB )

what this mean ?[/FONT]