For clearing nvram go to Commands under Administration and issue:
erase nvram && reboot
But if the USB stick worked then that won't have any effect. I was checking your partition map and it's kind of messy, this is mine:
Code:
--- /dev/sda
Block device, size 596.2 GiB (640135028736 bytes)
DOS/MBR partition map
Partition 1: 596.2 GiB (640132579328 bytes, 1250258944 sectors from 2048)
Type 0x07 (NTFS/HPFS)
Windows BOOTMGR boot loader
NTFS file system
UUID 22F266B3F2668ABD
Volume size 596.2 GiB (640132578816 bytes, 1250258943 sectors)
/dev/sda not mounted Unsupported Filesystem
--- /dev/sda1
Block device, size 596.2 GiB (640132579328 bytes)
Windows BOOTMGR boot loader
NTFS file system
UUID 22F266B3F2668ABD
Volume size 596.2 GiB (640132578816 bytes, 1250258943 sectors)
/dev/sda1 mounted to /tmp/mnt/sda1
I would try to re-partitioning the drive maybe.
PS. I have read you already did that, so the last thing I would try is flash one of the builds I told you. If that still doesn't work then maybe it has some issues with GPT. _________________ R6400v2 (boardID:30) - Kong 36480 running since 03/09/18 - (AP - DNSMasq - AdBlocking - QoS) R7800 - BS 31924 running since 05/26/17 - (AP - OpenVPN Client - DNSMasq - AdBlocking - QoS) R7000 - BS 30771 running since 12/16/16 - (AP - NAS - FTP - SMB - OpenVPN Server - Transmission - DDNS - DNSMasq - AdBlocking - QoS) R6250 - BS 29193 running since 03/20/16 - (AP - NAS - FTP - SMB - DNSMasq - AdBlocking)
I had exactly the same issue. 1Tb HDD used as secondary drive on win 10 machine with a single NTFS partition.
I attached the drive back to the pc and shrink the volume - created new NTFS partition about 4G. Then attached the drive back to the router with intention to try format the new partition within dd wrt and test. The new partition was auto mounted and I created new share of it (actually forgot to tick its select box in the user row) I tested the old/main partition and surprise - it was writable.