linux - mkfs - The device apparently does not exist; did you specify it correctly?

I used fdisk /dev/sda to create an sda6 partition. Now I want to format it using the mkfs.ext4 command. I am using CentOS 6.5 Linux distro. But when I run this command I get following error

mkfs.ext4
Could not stat /dev/sda6 --- No such file or directory
The device apparently does not exist; did you specify it correctly?

I am new to Linux and will appreciate any and all help. I have tried this with and without "sudo" and in both cases I get the same result.

[root@server1 ~]# sudo fdisk /dev/sda
WARNING: DOS-compatible mode is deprecated. It's strongly recommended to
         switch off the mode (command 'c') and change display units to
         sectors (command 'u').
Command (m for help): n
First cylinder (1530-3264, default 1530): 2042
Last cylinder, +cylinders or +size{K,M,G} (2042-3264, default 3264): +50MB
Command (m for help): p
Disk /dev/sda: 26.8 GB, 26843545600 bytes
255 heads, 63 sectors/track, 3263 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00002158
   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1         511     4096000   83  Linux
Partition 1 does not end on cylinder boundary.
/dev/sda2             511        1020     4096000   82  Linux swap / Solaris
Partition 2 does not end on cylinder boundary.
/dev/sda3            1020        1530     4096000   83  Linux
/dev/sda4            1530        3264    13925376    5  Extended
/dev/sda5            1531        2041     4096000   83  Linux
/dev/sda6            2042        2048       56227+  83  Linux
Command (m for help): w
The partition table has been altered!
Calling ioctl() to re-read partition table.
WARNING: Re-reading the partition table failed with error 16: Device or resource busy.
The kernel still uses the old table. The new table will be used at
the next reboot or after you run partprobe(8) or kpartx(8)
Syncing disks.
[root@server1 ~]# mkfs.ext4 /dev/sda6
mke2fs 1.41.12 (17-May-2010)
Could not stat /dev/sda6 --- No such file or directory
The device apparently does not exist; did you specify it correctly?

After restart - >

Command (m for help): p

Disk /dev/sda: 26.8 GB, 26843545600 bytes
255 heads, 63 sectors/track, 3263 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00002158

Device Boot Start End Blocks Id System
/dev/sda1 * 1 511 4096000 83 Linux
Partition 1 does not end on cylinder boundary.
/dev/sda2 511 1020 4096000 82 Linux swap / Solaris
Partition 2 does not end on cylinder boundary.
/dev/sda3 1020 1530 4096000 83 Linux
/dev/sda4 1530 3264 13925376 5 Extended
/dev/sda5 1531 2041 4096000 83 Linux
/dev/sda6 2042 2048 56227+ 83 Linux

Command (m for help): w
The partition table has been altered!

Calling ioctl() to re-read partition table.

WARNING: Re-reading the partition table failed with error 16: Device or resource busy.
The kernel still uses the old table. The new table will be used at
the next reboot or after you run partprobe(8) or kpartx(8)
Syncing disks.

[root@server1 ~]# partx /dev/sda

[root@server1 ~]# mkfs.ext4 /dev/sda6

mke2fs 1.41.12 (17-May-2010)

/dev/sda6 is apparently in use by the system; will not make a filesystem here!

[root@server1 ~]#

IMPORTANT
i noticed that fdisk /dev/sda - n - 6 - w, partx, mkfs.ext4 /dev/sda6 worked on red hat 6.5 rhel i have but is not working for CentOS 6.5. i don't know why.
Thanks