The diagnostics show what look like connection issues for /dev/sdg which I assume is disk5 and then it disappears.
Aug 1 00:01:06 Tower kernel: ata6.00: configured for UDMA/133 (device error ignored)
Aug 1 00:01:06 Tower kernel: sd 6:0:0:0: [sdg] tag#5 UNKNOWN(0x2003) Result: hostbyte=0x00 driverbyte=DRIVER_OK cmd_age=0s
Aug 1 00:01:06 Tower kernel: sd 6:0:0:0: [sdg] tag#5 Sense Key : 0x5 [current]
Aug 1 00:01:06 Tower kernel: sd 6:0:0:0: [sdg] tag#5 ASC=0x21 ASCQ=0x4
Aug 1 00:01:06 Tower kernel: sd 6:0:0:0: [sdg] tag#5 CDB: opcode=0x88 88 00 00 00 00 00 00 00 00 00 00 00 00 08 00 00
Aug 1 00:01:06 Tower kernel: I/O error, dev sdg, sector 0 op 0x0:(READ) flags 0x0 phys_seg 1 prio class 0
Aug 1 00:01:06 Tower kernel: Buffer I/O error on dev sdg, logical block 0, async page read
Aug 1 00:01:06 Tower kernel: ata6: EH complete
Aug 1 00:01:06 Tower emhttpd: error: ckmbr, 2291: Input/output error (5): read: /dev/sdg
Aug 1 00:01:06 Tower emhttpd: ckmbr error: -1
Aug 1 00:01:06 Tower kernel: mdcmd (6): import 5
Aug 1 00:01:06 Tower kernel: md: import_slot: 5 empty
I would suggest you carefully check that cabling to the drive (both power and SATA) is well seated and try again.