Having these lines in /var/log/syslog Apr 18 16:53:05 Server kernel: [4487878.816036] ata4: EH in SWNCQ mode,QC:qc_active 0x1 sactive 0x1 Apr 18 16:53:05 Server kernel: [4487878.816058] ata4: SWNCQ:qc_active 0x1 defer_bits 0x0 last_issue_tag 0x0 Apr 18 16:53:05 Server kernel: [4487878.816059] dhfis 0x1 dmafis 0x1 sdbfis 0x0 Apr 18 16:53:05 Server kernel: [4487878.816093] ata4: ATA_REG 0x40 ERR_REG 0x0 Apr 18 16:53:05 Server kernel: [4487878.816108] ata4: tag : dhfis dmafis sdbfis sacitve Apr 18 16:53:05 Server kernel: [4487878.816125] ata4: tag 0x0: 1 1 0 1 Apr 18 16:53:05 Server kernel: [4487878.816150] ata4.00: exception Emask 0x0 SAct 0x1 SErr 0x0 action 0x6 frozen Apr 18 16:53:05 Server kernel: [4487878.816178] ata4.00: failed command: WRITE FPDMA QUEUED Apr 18 16:53:05 Server kernel: [4487878.816199] ata4.00: cmd 61/08:00:00:88:e0/00:00:e8:00:00/40 tag 0 ncq 4096 out Apr 18 16:53:05 Server kernel: [4487878.816200] res 40/00:00:01:4f:c2/00:00:00:00:00/00 Emask 0x4 (timeout) Apr 18 16:53:05 Server kernel: [4487878.816253] ata4.00: status: { DRDY } Apr 18 16:53:05 Server kernel: [4487878.816272] ata4: hard resetting link Apr 18 16:53:05 Server kernel: [4487878.816274] ata4: nv: skipping hardreset on occupied port Apr 18 16:53:06 Server kernel: [4487879.676029] ata4: SATA link up 3.0 Gbps (SStatus 123 SControl 300) Apr 18 16:53:07 Server kernel: [4487880.416749] ata4.00: n_sectors mismatch 3907029168 != 268435455 Apr 18 16:53:07 Server kernel: [4487880.416752] ata4.00: revalidation failed (errno=-19) Apr 18 16:53:07 Server kernel: [4487880.416773] ata4.00: limiting speed to UDMA/133:PIO2 Apr 18 16:53:11 Server kernel: [4487884.676024] ata4: hard resetting link Apr 18 16:53:11 Server kernel: [4487884.676027] ata4: nv: skipping hardreset on occupied port Apr 18 16:53:12 Server kernel: [4487885.144032] ata4: SATA link up 3.0 Gbps (SStatus 123 SControl 300) Apr 18 16:53:12 Server kernel: [4487885.240185] ata4.00: failed to IDENTIFY (INIT_DEV_PARAMS failed, err_mask=0x80) Apr 18 16:53:12 Server kernel: [4487885.240190] ata4.00: revalidation failed (errno=-5) Apr 18 16:53:12 Server kernel: [4487885.240210] ata4.00: disabled Apr 18 16:53:17 Server kernel: [4487890.144023] ata4: hard resetting link Apr 18 16:53:17 Server kernel: [4487891.024033] ata4: SATA link up 3.0 Gbps (SStatus 123 SControl 300) Apr 18 16:53:17 Server kernel: [4487891.033357] ata4.00: ATA-8: WDC WD20EARS-00S8B1, 80.00A80, max UDMA/133 Apr 18 16:53:17 Server kernel: [4487891.033360] ata4.00: 3907029168 sectors, multi 1: LBA48 NCQ (depth 31/32) Apr 18 16:53:17 Server kernel: [4487891.048347] ata4.00: configured for UDMA/133 Apr 18 16:53:17 Server kernel: [4487891.048361] sd 3:0:0:0: [sdc] Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE Apr 18 16:53:17 Server kernel: [4487891.048365] sd 3:0:0:0: [sdc] Sense Key : Aborted Command [current] [descriptor] Apr 18 16:53:17 Server kernel: [4487891.048369] Descriptor sense data with sense descriptors (in hex): Apr 18 16:53:17 Server kernel: [4487891.048371] 72 0b 00 00 00 00 00 0c 00 0a 80 00 00 00 00 00 Apr 18 16:53:17 Server kernel: [4487891.048378] 00 00 00 00 Apr 18 16:53:17 Server kernel: [4487891.048382] sd 3:0:0:0: [sdc] Add. Sense: No additional sense information Apr 18 16:53:17 Server kernel: [4487891.048385] sd 3:0:0:0: [sdc] CDB: Write(10): 2a 00 e8 e0 88 00 00 00 08 00 Apr 18 16:53:17 Server kernel: [4487891.048393] end_request: I/O error, dev sdc, sector 3907028992 Apr 18 16:53:17 Server kernel: [4487891.048420] sd 3:0:0:0: rejecting I/O to offline device Apr 18 16:53:17 Server kernel: [4487891.048440] sd 3:0:0:0: rejecting I/O to offline device Apr 18 16:53:17 Server kernel: [4487891.048458] end_request: I/O error, dev sdc, sector 3907028992 Apr 18 16:53:17 Server kernel: [4487891.048477] md: super_written gets error=-5, uptodate=0 Apr 18 16:53:17 Server kernel: [4487891.048482] raid5: Disk failure on sdc, disabling device. Apr 18 16:53:17 Server kernel: [4487891.048483] raid5: Operation continuing on 3 devices. Apr 18 16:53:17 Server kernel: [4487891.048525] ata4: EH complete Apr 18 16:53:17 Server kernel: [4487891.048554] sd 3:0:0:0: rejecting I/O to offline device Apr 18 16:53:17 Server kernel: [4487891.048576] sd 3:0:0:0: rejecting I/O to offline device Apr 18 16:53:17 Server kernel: [4487891.048596] sd 3:0:0:0: rejecting I/O to offline device Apr 18 16:53:17 Server kernel: [4487891.048615] sd 3:0:0:0: [sdc] READ CAPACITY(16) failed Apr 18 16:53:17 Server kernel: [4487891.048617] sd 3:0:0:0: [sdc] Result: hostbyte=DID_NO_CONNECT driverbyte=DRIVER_OK Apr 18 16:53:17 Server kernel: [4487891.048620] sd 3:0:0:0: [sdc] Sense not available. Apr 18 16:53:17 Server kernel: [4487891.048624] sd 3:0:0:0: rejecting I/O to offline device Apr 18 16:53:17 Server kernel: [4487891.048643] sd 3:0:0:0: rejecting I/O to offline device Apr 18 16:53:17 Server kernel: [4487891.048663] sd 3:0:0:0: rejecting I/O to offline device Apr 18 16:53:17 Server kernel: [4487891.048681] sd 3:0:0:0: [sdc] READ CAPACITY failed Apr 18 16:53:17 Server kernel: [4487891.048683] sd 3:0:0:0: [sdc] Result: hostbyte=DID_NO_CONNECT driverbyte=DRIVER_OK Apr 18 16:53:17 Server kernel: [4487891.048685] sd 3:0:0:0: [sdc] Sense not available. Apr 18 16:53:17 Server kernel: [4487891.048689] sd 3:0:0:0: rejecting I/O to offline device Apr 18 16:53:17 Server kernel: [4487891.048709] sd 3:0:0:0: rejecting I/O to offline device Apr 18 16:53:17 Server kernel: [4487891.048800] sd 3:0:0:0: rejecting I/O to offline device Apr 18 16:53:17 Server kernel: [4487891.048860] sd 3:0:0:0: rejecting I/O to offline device Apr 18 16:53:17 Server kernel: [4487891.049028] sd 3:0:0:0: [sdc] Asking for cache data failed Apr 18 16:53:17 Server kernel: [4487891.049048] sd 3:0:0:0: [sdc] Assuming drive cache: write through Apr 18 16:53:17 Server kernel: [4487891.049071] sdc: detected capacity change from 2000398934016 to 0 Apr 18 16:53:17 Server kernel: [4487891.049080] ata4.00: detaching (SCSI 3:0:0:0) Apr 18 16:53:18 Server kernel: [4487891.061149] sd 3:0:0:0: [sdc] Stopping disk Apr 18 16:53:18 Server kernel: [4487891.485492] RAID5 conf printout: Apr 18 16:53:18 Server kernel: [4487891.485496] --- rd:4 wd:3 Apr 18 16:53:18 Server kernel: [4487891.485500] disk 0, o:1, dev:sdb Apr 18 16:53:18 Server kernel: [4487891.485502] disk 1, o:0, dev:sdc Apr 18 16:53:18 Server kernel: [4487891.485504] disk 2, o:1, dev:sdd Apr 18 16:53:18 Server kernel: [4487891.485506] disk 3, o:1, dev:sde Apr 18 16:53:18 Server kernel: [4487891.497014] RAID5 conf printout: Apr 18 16:53:18 Server kernel: [4487891.497016] --- rd:4 wd:3 Apr 18 16:53:18 Server kernel: [4487891.497018] disk 0, o:1, dev:sdb Apr 18 16:53:18 Server kernel: [4487891.497019] disk 2, o:1, dev:sdd Apr 18 16:53:18 Server kernel: [4487891.497021] disk 3, o:1, dev:sde Apr 18 16:53:18 Server kernel: [4487891.838719] scsi 3:0:0:0: Direct-Access ATA WDC WD20EARS-00S 80.0 PQ: 0 ANSI: 5 Apr 18 16:53:18 Server kernel: [4487891.838886] sd 3:0:0:0: Attached scsi generic sg3 type 0 Apr 18 16:53:18 Server kernel: [4487891.838911] sd 3:0:0:0: [sdf] 3907029168 512-byte logical blocks: (2.00 TB/1.81 TiB) Apr 18 16:53:18 Server kernel: [4487891.838964] sd 3:0:0:0: [sdf] Write Protect is off Apr 18 16:53:18 Server kernel: [4487891.838967] sd 3:0:0:0: [sdf] Mode Sense: 00 3a 00 00 Apr 18 16:53:18 Server kernel: [4487891.838988] sd 3:0:0:0: [sdf] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA Apr 18 16:53:20 Server kernel: [4487891.839147] sdf: unknown partition table Apr 18 16:53:20 Server kernel: [4487893.130026] sd 3:0:0:0: [sdf] Attached SCSI disk Right now, i'm unable to do anything on /dev/sdc. Is there any way to try to re-attach it? I don't want to power-down the server unless absolutely necessary System: Debian Stable 2.6.32-5-amd64 mdadm version 3.1.4-1+8efb9d1 cat /proc/mdstat Personalities : [raid6] [raid5] [raid4] md0 : active raid5 sdb[0] sdc[4](F) sde[3] sdd[2] 5860543488 blocks level 5, 64k chunk, algorithm 2 [4/3] [U_UU] unused devices: <none> mdadm --examine --scan ARRAY /dev/md0 UUID=1a7744b5:912ec7af:f82a9565:e3b453b4