I have checked your stickies, and i think now i understand the difference of RAID 1 and Standard Formatting. And i basically don't know why anyone would choose RAID 1.
RAID 1 provides redundancy, which is different from a backup. Redundancy reduces system downtime in the event of a HDD failure. If HDD1 fails, then HDD2 is immediately accessible without losing productivity. Under this scenario, if a full backup is also maintained on another physical device, the backup is
never used for daily work -- the sole purpose of the backup is to restore the RAID 1 array in the event the entire RAID 1 array is lost.
RAID 1 + full backup provides two layers of security.
If i use Standard Formatting, i will place all the data on Volume 1, and backup only the [HIGH] importance ones on Volume 2, using the Local Backups and SyncBack you mentioned. Therefore i will only be using Volume 1. I will not backup the data on my external hard disk often, due to big amount of data. Isn't it possible though that if one disk fails the other will too? I always found that having the disk turned off is the safest choice to guard your data. Is there any way that Volume 2, which will hold the data be turned off and on whenever i want?
If it gives you some peace of mind, I have two DNS-343s, and all 8 HDDs are formatted as Standard Volumes. The first DNS-343 backs up to the second DNS-343. I use Syncback once a week to ensure both DNS-343s are synchronized. For data that is "super critical" (if there is such a term), I have a tertiary backup to yet another device.
To answer your question regarding HDD failures, a failure of the first HDD should have no impact on the second HDD. Although there are exceptions to this rule, both HDDs are not likely to be lost unless there is physical damage to the DNS-325 itself (water damage, storm, electrical storm) or theft.
I'm not 100% sure how the hibernation rules work in the DNS-320 with respect to both HDDs, but in the DNS-343, HDDs that are not accessed stay spun down, reducing additional wear and tear. If this holds true with the DNS-320, then your backup HDD should remain idle unless you are actively reading/writing to/from the drive. You can test whether the drive is idle or not through physical inspection (i.e. listen for the HDD motor or feel the HDD for vibrations).