Raw Device Mapping (RDM) is a a method to pass a physical drive (that is detected by ESXi) to a virtual machine without first creating a Datastore and a virtual hard disk inside it.

Here is how to setup a drive with RDM to a VM:

1. Enable SSH on the host. Log in to the physical ESXi host. Under Troubleshooting Options select Enable SSH.
2. Log in to the host with your favorite SSH client
3. Find out what the disk is called by issuing the command:
   ls -l /vmfs/devices/disks/
   The device is called something like:
   t10.ATA_____SAMSUNG_HD103SJ___________S246JDWS90XXXX______
   Make sure you determine the correct drive to use for the RDM. Entries with the same beginning as above but ending with :1 is a partition. This is not what you want, you want to map the entire drive.
4. Find your datastore by issuing the command:
   ls -l /vmfs/volumes/
   It should be named something like:
   Datastore1 -> 509159-bd99-…
5. Go to your Datastore by issuing the command:
   cd /vmfs/volumes/Datastore1
   (Type ls and you will see the content of this Datastore)
6. Here comes the actual mapping. Issue the command:
   vmkfstools -z /vmfs/devices/disks/<name of disk from step 3> <RDM>.vmdk
   Where you replace the <name of disk from step 3> with the actual disk name and <RDM>with what you would like to call the RDM. A concrete example:vmkfstools -z /vmfs/devices/disks/t10.ATA_DISKNAME_ SamsungRDM.vmdk
7. Log in to the vSphere client
8. Shutdown the virtual machine that you want to add the RDM to
9. Open the settings for the virtual machine
10. Under hardware tab, click Add…
11. Select Hard Disk and press Next
12. Select Use an existing virutal disk
13. Press Browse, go to the datastore we found in step 4 and select theSamsungRDM.vmdk we created in step 6.
14. Press Next, Next and Finish to finalize the add hardware guide.

The hard drive is now added to the VM. Just start it up and start using the drive.

Note 1: I was trying to be a smart ass by using the path /dev/disks/ instead of /vmfs/devices/disks/ to point out the disk for the vmkfstools and it refused to accept it.

Note 2: SMART monitoring does not work on drives used as RDM.