The case is, you have an ESX server, and it have 1 local disk from server and more disk from SAN. So, if you want to give the OS VM have a new disk that the disk is not from datastore or the disk is RAW disk. This the step.
1. Assign lun to the ESX server.
2. Then rescan all lun.
3. Then, you go to the inventory, right click one host
4. Then Click edit setting so will show like this display
5. Click Add button on the top
6. Choose Add Harddisk - Choose Raw disk Mapping - next
7. After that you just click next Button - See this Picture
8. Finish after that check your server...