BackupSecurity

UmFriend is correct Plex only sees what DP presents to it as it sees the pool drive (virtual) and not the disks
if a drive fails then the "other" copy is used so Plex or any app carries on as normal if you have duplication on. As for the streaming error - could be a number of reasons - one of which is the file was damaged by the disk failure or the disk was removed and plex then could not find the file.......
If you have read striping on you can see in DP that it uses both disks at various times when presenting a file to application X - this does not radically change the speed of the read but if one disk slows (as other files are also being accessed at the same time on one disk) the other will be used etc etc. The speed of the read is the speed of the disk being used - no magic here.
Basically if you want redundancy then you need more space for the duplication or be selective in which shows you duplicate - but you will need more space at some point.
 

In the SMART details for that drive look for "Reallocated sector count", "Reallocation Event Count", "Uncorrectable Sector Count"... The Raw Values should be zero, if not that means there's some bad sectors. It's not always end of the world, if there's only a few. But it may be an indication why Scanner is maybe showing an issue. If those are all zero, then I'm not sure what else to look for. 
Does scanner show what that "1 warning" is anywhere? You'd think that it would show you somewhere what that "1 warning" is.
I'm fairly new to Stablebit Scanner myself, but hopefully you can figure this out somehow (and a Stablebit rep stops by too, hopefully). Otherwise I'd put in a ticket with them. I did ask a question to them about DrivePool and they responded within 24 hours.