Mount error, special device does not exist

I have a hard disk of 320 GB on ubuntu 12.04 64-bit.
2 drives of my hard (sda7 and sda8 of ext3 type) cannot be mounted.

output of sudo mount /dev/sda7 /home/newfolder -t ext3 :

mount: special device /dev/sda7 does not exist

/dev/ contains the following:

 sda   sda1   sda5   sda6

but GParted shows sda7 and sda8:

output of blkid:

/dev/sda1: UUID="a898f3ad-11d9-4dbb-9ea8-71a819dc8f70" TYPE="ext4" 
/dev/sda5: UUID="998c7c6f-5ff8-426c-83d4-1a309b7cdc4f" TYPE="swap" 
/dev/sda6: UUID="da0460d0-714e-40ae-b88b-a0deca87087c" TYPE="ext4" 
/dev/sdb1: LABEL="FLASH DRIVE" UUID="8A24-B5CD" TYPE="vfat"

output of fdisk -l:

Disk /dev/sda: 320.1 GB, 320071851520 bytes
255 heads, 63 sectors/track, 38913 cylinders, total 625140335 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x17ea17ea

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *        2048      194559       96256   83  Linux
/dev/sda2          196607   625121279   312462336+   f  W95 Ext'd (LBA)
/dev/sda5          196608     8007679     3905536   82  Linux swap / Solaris
/dev/sda6         8009728    61431807    26711040   83  Linux
/dev/sda7        61432623   337911209   138239293+  83  Linux
/dev/sda8       337911273   625121279   143605003+  83  Linux

output of cat /etc/fstab:

# /etc/fstab: static file system information.
#
# Use 'blkid' to print the universally unique identifier for a
# device; this may be used with UUID= as a more robust way to name devices
# that works even if disks are added and removed. See fstab(5).
#
# <file system> <mount point>   <type>  <options>       <dump>  <pass>
proc            /proc           proc    nodev,noexec,nosuid 0       0
# / was on /dev/sda8 during installation
UUID=da0460d0-714e-40ae-b88b-a0deca87087c /               ext4    errors=remount-ro 0       1
# /boot was on /dev/sda1 during installation
UUID=a898f3ad-11d9-4dbb-9ea8-71a819dc8f70 /boot           ext4    defaults        0       2
# swap was on /dev/sda7 during installation
UUID=998c7c6f-5ff8-426c-83d4-1a309b7cdc4f none            swap    sw              0       0
/dev/fd0        /media/floppy0  auto    rw,user,noauto,exec,utf8 0       0

output of ls /dev/mapper :

control

Did you re-partition that disk recently? That's the only thing that comes into my mind that could cause the kernel's understanding of the partition layout to differ from fdisk's.

If so, you need to tell the kernel to re-read the partition table with

sudo blockdev --rereadpt

(This used to be sfdisk -R, but sfdisk dropped -R in version 2.26.)

This can fail (with an error message) if you have any of the partitions of that disk already mounted. Unmount them and try again. Sometimes that's not possible (e.g. when your root partition is on the same disk), and you have to reboot instead.

(There is actually a way to force the kernel to re-read the partition table, which ought to be safe enough if you didn't change the sizes or positions of any of the mounted partition, but I don't remember the exact command line. I think it involves partx or kpartx.)

Check lv status:

lvscan

check if the INACTIVE same as what you are missing.

To fix do vgchange:

vgchange -ay