How to Get Expanded Synology Raid to be Recognized in Mac OS?
I figured this out a few days after posting my question, and figured I should followup for anyone else experiencing a similar problem. I came across gdisk and used the following instructions in Terminal to fix the issue:

* Type "sudo gdisk /dev/sdc" (or whatever the appropriate device filename is)
* Type "x" to enter experts' mode
* Type "e" to relocate the backup data
* Type "w" to save your changes.