Since you had the LUN added, have you re-scanned for devices (or rebooted).
This link talks about CentOS7, but it should be pretty much the same. Perhaps the command
kudzu might be an option too for RHEL5.
I find I always need to re-scan to get new/enlarged LUNs reflected in the OS.
I suspect that the LUN is not yet showing up, hence the confusion of the two disks you do have. I think that they are both local, one a simple disk, one a RAID device.
Robin