Strange things in dmesg, but not in the logs.

Having the same messages on a ReadyNAS 516 (with one EDA500) running latest firmware (6.7.4). Here's the full dmesg context:

[Jun 9 15:28] ata9: illegal qc_active transition (00000000->00000002)
[ +0.009918] ata9.00: failed to read SCR 1 (Emask=0x40)
[ +0.000003] ata9.01: failed to read SCR 1 (Emask=0x40)
[ +0.000001] ata9.02: failed to read SCR 1 (Emask=0x40)
[ +0.000001] ata9.03: failed to read SCR 1 (Emask=0x40)
[ +0.000001] ata9.04: failed to read SCR 1 (Emask=0x40)
[ +0.000001] ata9.05: failed to read SCR 1 (Emask=0x40)
[ +0.000005] ata9.15: exception Emask 0x2 SAct 0x0 SErr 0x0 action 0x6 frozen
[ +0.000001] ata9.00: exception Emask 0x100 SAct 0x0 SErr 0x0 action 0x6 frozen
[ +0.000002] ata9.01: exception Emask 0x100 SAct 0x0 SErr 0x0 action 0x6 frozen
[ +0.000001] ata9.02: exception Emask 0x100 SAct 0x0 SErr 0x0 action 0x6 frozen
[ +0.000001] ata9.02: failed command: WRITE DMA EXT
[ +0.000003] ata9.02: cmd 35/00:80:40:52:f7/00:00:91:00:00/e0 tag 1 dma 65536 out
res 50/00:00:bf:52:f7/00:00:91:00:00/e0 Emask 0x100 (unknown error)
[ +0.000002] ata9.02: status: { DRDY }
[ +0.000001] ata9.03: exception Emask 0x100 SAct 0x0 SErr 0x0 action 0x6 frozen
[ +0.000001] ata9.04: exception Emask 0x100 SAct 0x0 SErr 0x0 action 0x6 frozen
[ +0.000001] ata9.04: failed command: WRITE DMA EXT
[ +0.000003] ata9.04: cmd 35/00:80:c0:17:f7/00:00:91:00:00/e0 tag 0 dma 65536 out
res 50/00:00:3f:18:f7/00:00:91:00:00/e0 Emask 0x100 (unknown error)
[ +0.000001] ata9.04: status: { DRDY }
[ +0.000001] ata9.05: exception Emask 0x100 SAct 0x0 SErr 0x0 action 0x6 frozen
[ +0.000003] ata9.15: hard resetting link
[ +2.120968] ata9.15: SATA link up 3.0 Gbps (SStatus 123 SControl 0)
[ +0.000006] ata9.15: PMP revalidation failed (errno=-19)
[ +4.999743] ata9.15: hard resetting link
[ +2.132041] ata9.15: SATA link up 3.0 Gbps (SStatus 123 SControl 0)
[ +0.000577] ata9.00: hard resetting link
[ +0.304716] ata9.00: SATA link up 3.0 Gbps (SStatus 123 SControl 320)
[ +0.000031] ata9.01: hard resetting link
[ +0.304933] ata9.01: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
[ +0.000031] ata9.02: hard resetting link
[ +0.305354] ata9.02: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
[ +0.000074] ata9.03: hard resetting link
[ +0.305745] ata9.03: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
[ +0.000057] ata9.04: hard resetting link
[ +0.304861] ata9.04: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
[ +0.000031] ata9.05: hard resetting link
[ +0.304778] ata9.05: SATA link up 1.5 Gbps (SStatus 113 SControl 320)
[ +0.010022] ata9.00: configured for UDMA/100
[ +0.009711] ata9.01: configured for UDMA/100
[ +0.009050] ata9.02: configured for UDMA/100
[ +0.007537] ata9.03: configured for UDMA/100
[ +0.009999] ata9.04: configured for UDMA/100
[ +0.000074] ------------[ cut here ]------------
[ +0.000005] WARNING: CPU: 3 PID: 1051 at drivers/ata/libata-eh.c:3993 ata_eh_finish+0xcd/0xd0()
[ +0.000001] Modules linked in: sx8635(O) vpd(PO)
[ +0.000004] CPU: 3 PID: 1051 Comm: scsi_eh_8 Tainted: P W O 4.4.68.x86_64.1 #1
[ +0.000001] Hardware name: NETGEAR ReadyNAS 516/ReadyNAS 516, BIOS 4.6.5 04/16/2013
[ +0.000001] 0000000000000000 ffff8800d5153c40 ffffffff883d3e98 0000000000000000
[ +0.000002] ffffffff88d32e85 ffff8800d5153c78 ffffffff8806b27c ffff880119df5d40
[ +0.000001] ffff880119df5d40 ffff880119df4000 ffff880119df4000 ffff880119df7480
[ +0.000001] Call Trace:
[ +0.000003] [<ffffffff883d3e98>] dump_stack+0x4d/0x65
[ +0.000002] [<ffffffff8806b27c>] warn_slowpath_common+0x7c/0xb0
[ +0.000002] [<ffffffff8806b365>] warn_slowpath_null+0x15/0x20
[ +0.000001] [<ffffffff8861df6d>] ata_eh_finish+0xcd/0xd0
[ +0.000002] [<ffffffff88620730>] sata_pmp_error_handler+0x630/0xa20
[ +0.000001] [<ffffffff88613950>] ? ata_phys_link_offline+0x30/0x30
[ +0.000002] [<ffffffff88627e20>] ? sil24_pmp_attach+0x70/0x70
[ +0.000002] [<ffffffff886274a0>] ? sil24_pmp_hardreset+0x60/0x60
[ +0.000001] [<ffffffff886137f0>] ? ata_phys_link_online+0x30/0x30
[ +0.000002] [<ffffffff8809613c>] ? set_next_entity+0x6c/0x7d0
[ +0.000002] [<ffffffff8862742a>] sil24_error_handler+0x2a/0x40
[ +0.000002] [<ffffffff8861e3a0>] ata_scsi_port_error_handler+0x430/0x770
[ +0.000001] [<ffffffff8861e76c>] ata_scsi_error+0x8c/0xc0
[ +0.000003] [<ffffffff885c3186>] scsi_error_handler+0xa6/0x530
[ +0.000002] [<ffffffff88a4d184>] ? __schedule+0x244/0x6f0
[ +0.000001] [<ffffffff885c30e0>] ? scsi_eh_get_sense+0x150/0x150
[ +0.000002] [<ffffffff885c30e0>] ? scsi_eh_get_sense+0x150/0x150
[ +0.000002] [<ffffffff88085096>] kthread+0xe6/0x100
[ +0.000002] [<ffffffff88084fb0>] ? kthread_create_on_node+0x170/0x170
[ +0.000002] [<ffffffff88a5095f>] ret_from_fork+0x3f/0x70
[ +0.000001] [<ffffffff88084fb0>] ? kthread_create_on_node+0x170/0x170
[ +0.000001] ---[ end trace b98d4729235a7b6e ]---
[ +0.000006] ata9: EH complete

I don't remember seing those before a few days ago, I believe (but don't take my word for it).

Cheers.