RN516 hangs after reboot / won't mount filesystem

Hi all,

My RN516 running ReadyNAS OS 6.2.4 with 6 x 3TB WD RED NAS Drives has decided to start hanging on boot, it still manages to get as far as letting me ssh in to discover this message in dmesg:

INFO: task btrfs-transacti:2339 blocked for more than 120 seconds.
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
btrfs-transacti D 0000000000000000 0 2339 2 0x00000000
ffff8801053c9d20 0000000000000046 0000000000000001 ffff88011643a3a8
00000027c8f5d0c8 ffff8801053c9fd8 ffff8801053c9fd8 0000000000004000
ffff88011a86b480 ffff88011643a0d0 00000001053c9c60 ffffffff880d1a3e
Call Trace:
[<ffffffff880d1a3e>] ? deactivate_slab+0x4e/0xf0
[<ffffffff882ba535>] ? btrfs_alloc_path+0x15/0x20
[<ffffffff880d225e>] ? kmem_cache_alloc+0xde/0xe0
[<ffffffff882ba535>] ? btrfs_alloc_path+0x15/0x20
[<ffffffff880d2445>] ? kmem_cache_free+0x15/0xa0
[<ffffffff8888959a>] schedule+0x3a/0x50
[<ffffffff88889aa9>] schedule_timeout+0x1a9/0x210
[<ffffffff88064649>] ? prepare_to_wait+0x59/0x90
[<ffffffff882e177b>] btrfs_commit_transaction+0x20b/0xa90
[<ffffffff88064850>] ? abort_exclusive_wait+0xb0/0xb0
[<ffffffff882e2675>] ? start_transaction+0x95/0x410
[<ffffffff882d98e5>] transaction_kthread+0x255/0x270
[<ffffffff882d9690>] ? btrfs_alloc_root+0x40/0x40
[<ffffffff88063f89>] kthread+0x89/0x90
[<ffffffff8888ccf4>] kernel_thread_helper+0x4/0x10
[<ffffffff88063f00>] ? kthread_worker_fn+0x140/0x140
[<ffffffff8888ccf0>] ? gs_change+0xb/0xb

I can see from cat /proc/mdstat that the raid looks ok?

root@nas:/var/log# cat /proc/mdstat
Personalities : [raid0] [raid1] [raid10] [raid6] [raid5] [raid4]
md127 : active raid5 sdc3[0] sda3[5] sdb3[4] sdf3[3] sde3[2] sdd3[1]
14627073280 blocks super 1.2 level 5, 64k chunk, algorithm 2 [6/6] [UUUUUU]

md1 : active raid6 sdc2[0] sda2[5] sdb2[4] sdf2[3] sde2[2] sdd2[1]
2095104 blocks super 1.2 level 6, 512k chunk, algorithm 2 [6/6] [UUUUUU]

md0 : active raid1 sdc1[0] sda1[5] sdb1[4] sdf1[3] sde1[2] sdd1[1]
4192192 blocks super 1.2 [6/6] [UUUUUU]

later in dmesg I see this fairly serious message?

kernel BUG at fs/btrfs/extent-tree.c:1566!
invalid opcode: 0000 [#1] SMP
CPU 3
Modules linked in: ir_rc6_decoder ir_rc5_decoder nuvoton_cir ir_nec_decoder rc_core sx8635 rn_gpio vpd(P)

Pid: 1485, comm: mount Tainted: P 3.0.101.RNx86_64.3 #1 NETGEAR ReadyNAS 516/ReadyNAS 516
RIP: 0010:[<ffffffff882c5760>] [<ffffffff882c5760>] lookup_inline_extent_backref+0x4c0/0x4f0
RSP: 0018:ffff880109833618 EFLAGS: 00010246
RAX: 0000000000ffff88 RBX: ffff8800a7546138 RCX: ffff8800a67dc000
RDX: 0000000000000000 RSI: 00000000000009c2 RDI: ffff8800a7546138
RBP: ffff8801098336b8 R08: 0000000000001000 R09: ffff8801098335c8
R10: 0000000000000000 R11: 0000000000000000 R12: 00000000000000b2
R13: 000000000000094d R14: 00000000000009ca R15: 00000000000009e7
FS: 00007f02eccb3800(0000) GS:ffff88011f380000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: 00007f245d62e080 CR3: 00000001158cf000 CR4: 00000000001407e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Process mount (pid: 1485, threadinfo ffff880109832000, task ffff8801158520d0)
Stack:
ffffffff882ba535 ffff880115d13540 ffff880109833748 000004c1a73df000
ffff8800acf0a840 ffff88011f3978d0 0000000000000035 ffffffff00008050
ffff88011aaa6000 ffff8800acf0a900 000000000011d3f2 00ffffff882ba535
Call Trace:
[<ffffffff882ba535>] ? btrfs_alloc_path+0x15/0x20
[<ffffffff882c87d4>] __btrfs_free_extent+0xb4/0x8b0
[<ffffffff88326384>] ? btrfs_merge_delayed_refs+0x204/0x3c0
[<ffffffff8832656b>] ? btrfs_check_delayed_seq+0x2b/0xb0
[<ffffffff882cc5e0>] run_clustered_refs+0x3c0/0xe40
[<ffffffff882cc6c8>] ? run_clustered_refs+0x4a8/0xe40
[<ffffffff883266d8>] ? btrfs_find_ref_cluster+0xe8/0x170
[<ffffffff882d0a00>] btrfs_run_delayed_refs+0xe0/0x5a0
[<ffffffff882e15f5>] btrfs_commit_transaction+0x85/0xa90
[<ffffffff88064850>] ? abort_exclusive_wait+0xb0/0xb0
[<ffffffff880d24ca>] ? kmem_cache_free+0x9a/0xa0
[<ffffffff8831e27d>] btrfs_recover_log_trees+0x3bd/0x490
[<ffffffff8831c4e0>] ? replay_one_dir_item+0x100/0x100
[<ffffffff882de873>] open_ctree+0x1883/0x2250
[<ffffffff8813a1fa>] ? disk_name+0xaa/0xb0
[<ffffffff882b6c7b>] btrfs_mount+0x50b/0x710
[<ffffffff880d9710>] mount_fs+0x40/0x1b0
[<ffffffff880b7ecb>] ? __alloc_percpu+0xb/0x10
[<ffffffff880f3097>] vfs_kern_mount+0x67/0xd0
[<ffffffff880f34f1>] do_kern_mount+0x51/0x100
[<ffffffff880f4cc3>] do_mount+0x253/0x860
[<ffffffff880b36c4>] ? strndup_user+0x54/0x70
[<ffffffff880f56c5>] sys_mount+0x95/0xf0
[<ffffffff8888bf7b>] system_call_fastpath+0x16/0x1b
Code: 55 30 85 d2 4c 89 31 74 89 e9 2d ff ff ff 0f 1f 44 00 00 b8 0d 00 00 00 e9 32 fd ff ff 66 0f 1f 44 00 00 a8 01 0f 85 e3 fc ff ff <0f> 0b 66 0f 1f 44 00 00 c7 45 9c 0d 00 00 00 e9 a3 fb ff ff b8
RIP [<ffffffff882c5760>] lookup_inline_extent_backref+0x4c0/0x4f0
RSP <ffff880109833618>

Trying to mount /dev/md127 results in the command hanging.

Is there anything that can be done or should I setup a 2nd machine and attempt to mount the array there and copy the contents over?

Thanks izy