I don't see a way to get the information for racadm. 
Hi! If I boot to BIOS setup and go to iDrac settings -> System summary when in the bottom I can see section "Backplane inventory" with row "Hard drive form factor" ( 2.5'' or 3.5''). Is it possible to get this information via racadm or wsman?   [IMAGE]
Get backplane hard drive form factor