I have a T420 Full model number: 4180-PR1, and have a 320 GB HDD, but it's nearly full so I am thinking about buying an SSD.
I wanted to know is it possible to have 1 SSD and keep the existing 320GB HDD and run it alongside each other.v
Yes you can, and there are two ways to do it.
1. Buy a mSATA SSD for the OS and put it in your WWAN slot, keeping your old HD where it is for data.
2. Get a 2.5" SSD for OS, swap your old HD out for it and put the old HD in a Ultrabay caddy and swap out your CD/DVD drive for that. It can be hotswapped if you need to use the CD/DVD drive.
Some community users shared that the HDD can be set to spin down when idle so the machine runs cool and quiet. Another thing users can do is to put a bootable Ubuntu and bootable Acronis on the HDD and take regular backups of the SSD and store the image on the HDD, and also make backups of both to an external drive.
To set the drive to spin down:
Control panel -> (Power options) change when the computer sleeps -> Advanced Settings -> Hard Disk -> turn off disk after...