I have a Storage Pool setup using Windows Storage Spaces that I use to backup to. Since the Windows 10 FCU 1709 update, if I boot to USB, the Macrium Restore is unable to access or even see that the Storage Pool drives exist. This worked in the past. Tried creating new USB Boot drive, but still unable.

So just updating to Win10 1709 caused this?  You didn't reconfigure your Storage Space after the update at all?  In that case, I guess it's still possible that Win10 1709 did something automatically in the background that rendered your Storage Space incompatible with Win10 1607, which is the most recent WinPE version that Reflect currently supports.

No, I did not reconfigure it. It may have updated something automatically.

I did not realize that Macrium Reflect only supported up to Win10 1607.

Sorry, my previous post wasn't very clear.  Macrium Reflect supports newer versions of Windows 10 just fine.  However, the Rescue Media that it builds runs on top of Windows PE, which is essentially a stripped down version of Windows primarily intended for performing and recovering Windows installations.  The newest WinPE version that can be used for Rescue Media is WinPE 10 1607.  So if some new Storage Spaces version was introduced for Windows 10 1709 that isn't backward compatible, Reflect running in Rescue wouldn't support it until Macrium adds support for building Rescue Media that runs on a version of WinPE that supports the new Storage Spaces version.  Something similar happened with BitLocker.  Windows 10 1511 added support for a new BitLocker "XTS" mode, and consequently, Rescue Media built on the original WinPE 10 1507 version (and all earlier versions) could not unlock BitLocker volumes that used that mode.  That limitation was removed when Macrium added support for building Rescue Media using WinPE 10 1511.