I have a Western Digital My passport external USB hard disk of size 5TB , with dedicated power supply

When I connect this external drive to my modem router usb 3 slot, connection is OK and the modem *does* read (on the usb share tab page) it's 4.5TB size

But,

It doesn't see the drive content and i see "Available 0G" on the USB storage device page of the management interface. even if re-booting, re-scanning in vain.

HDD is with external power source

5TB in single NTFS partition

Format is GPT

Does anyone know what is the largest size hard drive supported in the USB 3.0 port on the VR600 V. 3 ?
Does restructing my HDD into an upto 2TB partitions would solve the limitation?

How Should i troubleshoot this issue ?