This seems to be the problem:

Sep 15 11:02:07 lubuntu kernel: [  566.424843] XFS (sda11): bad magic number
Sep 15 11:02:07 lubuntu kernel: [  566.424861] ffff880091f1b000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
Sep 15 11:02:07 lubuntu kernel: [  566.424863] ffff880091f1b010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
Sep 15 11:02:07 lubuntu kernel: [  566.424876] ffff880091f1b020: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
Sep 15 11:02:07 lubuntu kernel: [  566.424880] ffff880091f1b030: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
Sep 15 11:02:07 lubuntu kernel: [  566.424887] XFS (sda11): Internal error xfs_sb_read_verify at line 780 of file /build/buildd/linux-3.11.0/fs/xfs/xfs_mount.c.  Caller 0xffffffffa081bb75
Sep 15 11:02:07 lubuntu kernel: [  566.424887] 
Sep 15 11:02:07 lubuntu kernel: [  566.424896] CPU: 1 PID: 21 Comm: kworker/1:0H Tainted: GF        C   3.11.0-7-generic #13-Ubuntu
Sep 15 11:02:07 lubuntu kernel: [  566.424899] Hardware name: GOOGLE Link, BIOS          12/10/2012
Sep 15 11:02:07 lubuntu kernel: [  566.424938] Workqueue: xfslogd xfs_buf_iodone_work [xfs]
Sep 15 11:02:07 lubuntu kernel: [  566.424945]  0000000000000001 ffff880149375d68 ffffffff816f2f51 ffff880146f97800
Sep 15 11:02:07 lubuntu kernel: [  566.424952]  ffff880149375d80 ffffffffa081e25b ffffffffa081bb75 ffff880149375db8
Sep 15 11:02:07 lubuntu kernel: [  566.424959]  ffffffffa081e2b5 0000030c00000000 ffff88007fbb8300 0000000000000016
Sep 15 11:02:07 lubuntu kernel: [  566.424965] Call Trace:
Sep 15 11:02:07 lubuntu kernel: [  566.424980]  [<ffffffff816f2f51>] dump_stack+0x45/0x56
Sep 15 11:02:07 lubuntu kernel: [  566.425012]  [<ffffffffa081e25b>] xfs_error_report+0x3b/0x40 [xfs]
Sep 15 11:02:07 lubuntu kernel: [  566.425041]  [<ffffffffa081bb75>] ? xfs_buf_iodone_work+0x85/0xf0 [xfs]
Sep 15 11:02:07 lubuntu kernel: [  566.425071]  [<ffffffffa081e2b5>] xfs_corruption_error+0x55/0x80 [xfs]
Sep 15 11:02:07 lubuntu kernel: [  566.425121]  [<ffffffffa0870413>] xfs_sb_read_verify+0x113/0x130 [xfs]
Sep 15 11:02:07 lubuntu kernel: [  566.425150]  [<ffffffffa081bb75>] ? xfs_buf_iodone_work+0x85/0xf0 [xfs]
Sep 15 11:02:07 lubuntu kernel: [  566.425177]  [<ffffffffa081bb75>] xfs_buf_iodone_work+0x85/0xf0 [xfs]
Sep 15 11:02:07 lubuntu kernel: [  566.425187]  [<ffffffff8107cfbc>] process_one_work+0x17c/0x430
Sep 15 11:02:07 lubuntu kernel: [  566.425193]  [<ffffffff8107dc0c>] worker_thread+0x11c/0x3c0
Sep 15 11:02:07 lubuntu kernel: [  566.425200]  [<ffffffff8107daf0>] ? manage_workers.isra.24+0x2a0/0x2a0
Sep 15 11:02:07 lubuntu kernel: [  566.425209]  [<ffffffff81084710>] kthread+0xc0/0xd0
Sep 15 11:02:07 lubuntu kernel: [  566.425217]  [<ffffffff81084650>] ? kthread_create_on_node+0x120/0x120
Sep 15 11:02:07 lubuntu kernel: [  566.425228]  [<ffffffff81702d2c>] ret_from_fork+0x7c/0xb0
Sep 15 11:02:07 lubuntu kernel: [  566.425236]  [<ffffffff81084650>] ? kthread_create_on_node+0x120/0x120
Sep 15 11:02:07 lubuntu kernel: [  566.425241] XFS (sda11): Corruption detected. Unmount and run xfs_repair
Sep 15 11:02:07 lubuntu kernel: [  566.425262] XFS (sda11): SB validate failed with error 22.
Sep 15 11:02:07 lubuntu ubiquity: mount: /dev/sda6: can't read superblock