You have disk errors:
Feb 26 11:06:20 Tower kernel: scsi_io_completion_action: 812 callbacks suppressed
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2596 UNKNOWN(0x2003) Result: hostbyte=0x00 driverbyte=DRIVER_OK cmd_age=0s
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2596 Sense Key : 0x2 [current]
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2596 ASC=0x4 ASCQ=0x0
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2596 CDB: opcode=0x88 88 00 00 00 00 05 10 38 4a f0 00 00 00 08 00 00
Feb 26 11:06:20 Tower kernel: blk_print_req_error: 812 callbacks suppressed
Feb 26 11:06:20 Tower kernel: I/O error, dev sdd, sector 21746961136 op 0x0:(READ) flags 0x80700 phys_seg 1 prio class 3
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2597 UNKNOWN(0x2003) Result: hostbyte=0x00 driverbyte=DRIVER_OK cmd_age=0s
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2597 Sense Key : 0x2 [current]
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2597 ASC=0x4 ASCQ=0x0
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2597 CDB: opcode=0x88 88 00 00 00 00 05 10 38 4a f0 00 00 00 08 00 00
Feb 26 11:06:20 Tower kernel: I/O error, dev sdd, sector 21746961136 op 0x0:(READ) flags 0x0 phys_seg 1 prio class 3
Feb 26 11:06:20 Tower kernel: buffer_io_error: 401 callbacks suppressed
Feb 26 11:06:20 Tower kernel: Buffer I/O error on dev sdd, logical block 2718370142, async page read
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2598 UNKNOWN(0x2003) Result: hostbyte=0x00 driverbyte=DRIVER_OK cmd_age=0s
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2598 Sense Key : 0x2 [current]
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2598 ASC=0x4 ASCQ=0x0
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2598 CDB: opcode=0x88 88 00 00 00 00 05 10 38 50 e0 00 00 00 08 00 00
Feb 26 11:06:20 Tower kernel: I/O error, dev sdd, sector 21746962656 op 0x0:(READ) flags 0x80700 phys_seg 1 prio class 3
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2599 UNKNOWN(0x2003) Result: hostbyte=0x00 driverbyte=DRIVER_OK cmd_age=0s
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2599 Sense Key : 0x2 [current]
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2599 ASC=0x4 ASCQ=0x0
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2599 CDB: opcode=0x88 88 00 00 00 00 05 10 38 50 e0 00 00 00 08 00 00
Feb 26 11:06:20 Tower kernel: I/O error, dev sdd, sector 21746962656 op 0x0:(READ) flags 0x0 phys_seg 1 prio class 3
Feb 26 11:06:20 Tower kernel: Buffer I/O error on dev sdd, logical block 2718370332, async page read
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2602 UNKNOWN(0x2003) Result: hostbyte=0x00 driverbyte=DRIVER_OK cmd_age=0s
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2602 Sense Key : 0x2 [current]
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2602 ASC=0x4 ASCQ=0x0
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2602 CDB: opcode=0x88 88 00 00 00 00 05 10 38 56 d0 00 00 00 08 00 00
Feb 26 11:06:20 Tower kernel: I/O error, dev sdd, sector 21746964176 op 0x0:(READ) flags 0x80700 phys_seg 1 prio class 3
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2603 UNKNOWN(0x2003) Result: hostbyte=0x00 driverbyte=DRIVER_OK cmd_age=0s
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2603 Sense Key : 0x2 [current]
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2603 ASC=0x4 ASCQ=0x0
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2603 CDB: opcode=0x88 88 00 00 00 00 05 10 38 56 d0 00 00 00 08 00 00
Feb 26 11:06:20 Tower kernel: I/O error, dev sdd, sector 21746964176 op 0x0:(READ) flags 0x0 phys_seg 1 prio class 3
Feb 26 11:06:20 Tower kernel: Buffer I/O error on dev sdd, logical block 2718370522, async page read
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2604 UNKNOWN(0x2003) Result: hostbyte=0x00 driverbyte=DRIVER_OK cmd_age=0s
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2604 Sense Key : 0x2 [current]
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2604 ASC=0x4 ASCQ=0x0
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2604 CDB: opcode=0x88 88 00 00 00 00 05 10 38 5c c0 00 00 00 08 00 00
Feb 26 11:06:20 Tower kernel: I/O error, dev sdd, sector 21746965696 op 0x0:(READ) flags 0x80700 phys_seg 1 prio class 3
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2605 UNKNOWN(0x2003) Result: hostbyte=0x00 driverbyte=DRIVER_OK cmd_age=0s
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2605 Sense Key : 0x2 [current]
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2605 ASC=0x4 ASCQ=0x0
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2605 CDB: opcode=0x88 88 00 00 00 00 05 10 38 5c c0 00 00 00 08 00 00
Feb 26 11:06:20 Tower kernel: I/O error, dev sdd, sector 21746965696 op 0x0:(READ) flags 0x0 phys_seg 1 prio class 3
Feb 26 11:06:20 Tower kernel: Buffer I/O error on dev sdd, logical block 2718370712, async page read
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2606 UNKNOWN(0x2003) Result: hostbyte=0x00 driverbyte=DRIVER_OK cmd_age=0s
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2606 Sense Key : 0x2 [current]
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2606 ASC=0x4 ASCQ=0x0
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2606 CDB: opcode=0x88 88 00 00 00 00 05 10 38 62 b0 00 00 00 08 00 00
Feb 26 11:06:20 Tower kernel: I/O error, dev sdd, sector 21746967216 op 0x0:(READ) flags 0x80700 phys_seg 1 prio class 3
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2607 UNKNOWN(0x2003) Result: hostbyte=0x00 driverbyte=DRIVER_OK cmd_age=0s
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2607 Sense Key : 0x2 [current]
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2607 ASC=0x4 ASCQ=0x0
Feb 26 11:06:20 Tower kernel: sd 7:0:2:0: [sdd] tag#2607 CDB: opcode=0x88 88 00 00 00 00 05 10 38 62 b0 00 00 00 08 00 00
Feb 26 11:06:20 Tower kernel: I/O error, dev sdd, sector 21746967216 op 0x0:(READ) flags 0x0 phys_seg 1 prio class 3
Feb 26 11:06:20 Tower kernel: Buffer I/O error on dev sdd, logical block 2718370902, async page read
Feb 26 11:06:20 Tower kernel: Buffer I/O error on dev sdd, logical block 2718371092, async page read
Feb 26 11:06:20 Tower kernel: Buffer I/O error on dev sdd, logical block 2718371282, async page read
Feb 26 11:06:20 Tower kernel: Buffer I/O error on dev sdd, logical block 2718371472, async page read
Feb 26 11:06:20 Tower kernel: Buffer I/O error on dev sdd, logical block 2718371662, async page read
Feb 26 11:06:20 Tower kernel: Buffer I/O error on dev sdd, logical block 2718371852, async page read
Feb 26 11:06:22 Tower kernel: sd 7:0:2:0: Power-on or device reset occurred
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Feb 26 11:06:24 Tower kernel: sd 7:0:15:0: Power-on or device reset occurred
Things to look at:
Power supply.
Cabling - power and/or data.
If you're running three drives at once, I'd suspect a power supply issue.