The /dev/sda in Linux means and refers to the hard disk or disk in the web server or Linux system.
The disk names in Linux are alphabetical and the /dev/sda is the first hard drive (the primary master), /dev/sdb is the second etc.
The numbers refer to the partitions, so /dev/sda1 is the first partition of the first drive.
Your external drive will not be /dev/sda, that is more likely to be the internal drive of your machine.
Basically the /dev/sda in Linux means the internal drive of the the server.
If your operating a web server and you notice the /dev/sda in your control panel it is referring to your internal hard drive and it should show up much space you have left on that hard drive.
If you're running out of space on the /dev/sda in Linux operating system or Centos Operating system then you need to order more space and upgrade the server or if you own the serve you need to install a bigger internal hard drive.