What is the physical size (thickness) of drive of a should I purchase to upgrade the main hard disk drive or solid state drive in a T420? I've heard there are 7mm thick drives and 9.5mm thick drives. How do I know which I have and which I can use? If I change from one to the other, what else do I need to change to mount it?
The T420 supports 2.5" mobile drives. However, these drives come in both 7mm and 9.5mm thickness. Versions of the T420 was orginally shipped from Lenovo with both kinds of drives. There are two different rubber shock mount designs to accomodate the two different drive types. Be sure to use the correct set of shock mounts when installing the drive. The 9.5mm mounts installed on a 7mm drive will be loose and sloppy and 7mm mounts will not install on a 9.5mm drive.
The simplest course of action is to purchase an upgrade drive that is the same thickness as the drive currently in the system. To verify the current drive's size, one can remove the drive and examine the drive bumpers. In the following photo the bumpers at the top are for a 9.5mm drive, and the bumpers at the bottom are for a 7mm drive.
When installing a drive that does match the currently installed thickness the set of drive bumpers should be acquired. These can be ordered through Lenovo service parts. Consult the HMM (Hardware maintenance manual) for your system found on the support.lenovo.com website.