Does the ThinkPad T420s use a 2.5" hard drive or a 1.8" hard drive? When considering upgraded capacity drives, the physical drive package - whether 1.8" or 2.5" and whether 7mm or 9.5mm in thickness is important to know.
The ThinkPad T420s uses / requires a 2.5", 7mm high drive. The PSREF sheet specs for both SSD and conventional rotating media drives:
Some: Solid State Drive (SSD) / SATA 3.0Gb/s, 2.5" wide, 7mm high Some: Hard Disk Drive / SATA 3.0Gb/s, 2.5" wide, 7mm high, Active Protection System™
Note that the standard T420 model accepts 7mm or 9.5mm thick drives.