Let me show you how :
- Open your esx server console and choose server that you need to add another HDD.
- Edit server than choose HDD tab than click add
- follow the instruction to suit your need.
- after add click finish.
Now you need to format the HDD. Below will show you how.
- ssh or open console to the server you add the HDD
- as root run sysinstall
- than choose label (label disk editor)
- now you need to create slice for you disk.Currently I choose auto to create the disk
- after you name the disk than choose "w"
- that all mate.
Have a good day ahead.