Overview
The IBM x3550 has been revised over four different generations, M1-M4. Pay very close attention to your machine type (such as 7978) which will help determine the generation. Drive options and trays changed with each iteration of the x3550. Some servers support LFF and SFF drives, others only supported SFF drives.
New generations of the x3550 correlate to new Intel CPU architecture updates. In most cases, CPUs use different sockets between each generation of server so upgrades are limited.