Pritam,

Be guided as below:

1. Check if your centos has 'gparted' installed 
2. If it is not installed you have to install it from EPEL -
https://fedoraproject.org/wiki/EPEL

3. Run gparted and check if your external disks are available. Check for
options in the top right dropdown box in the gparted window.

4. Try rebooting the computer and check again.

5. If you don't find your disks there - probably the digits are lost !

6. There are a lot of things you can do with gparted - but be sure what
you are doing and which disk is being treated ! These are linux things &
you cannot be complacent, like while you use chkdsk in windows.
Remember when you run gparted you are prompted to be at 'root' already.
Things once clicked may not be undone.

Wish you get your bytes... 

ArK