  Right, I was thinking that drive 0:2 would be in Span 1. I checked the Alert Logs in OMSA and see several "Predictive Failure reported: Physical Disk 0:2 Controller 0, Connector 0" events logged. These stopped a few months ago, it appears this confirms the disk 0:2 is in the server but is now not visible because it has failed, correct?   If the Alert Logs in OMSA were showing that PD 0:2 was in a predictive failed state, then it has to be replaced anyways. Note that failure of the remaining drive on Span 1 will lead to complete failure of the array.   I believe the drive counts depends on the SATA cable connection because they are enumerated. The drive will take on a count enumerated on the cable connected to it.   This is another way, The Server Has To Be Powered Down. But you will have to disconnect the cables and then remove the drive to be able to read off the serial number and firmly reconnect back
This server is off site so I haven't gotten the opportunity to physically inspect it. This is the cause for some of these questions. OMSA is installed on the server. Through that, I can see this info: It is using a PERC S100 controller. There is one Virtual Disk. Configured as RAID 10. State is degraded. There are 3 hard drives visible. Split between Span 0 and Span 1. In Span 0, the drives visible are 0:0 and 0:1. In Span 1: the drive visible is 0:3. My questions are: 1.  Is it accurate to say that a drive 0:2 exists but is not visible in OMSA because it has failed? 2. How would I be able to identify drive 0:2 when opening the server? Are there any indicator lights to check? 3. If I replace the failed drive 0:2 with a new drive (with the same specs), will the controller automatically add the new drive to the existing RAID 10 Virtual Disk?  
PE T110: RAID 10 Degraded - How to identify and replace drive?