How do we not know which disk will be formatted? Doesn't make sense for a "random" disk or both disks to be formatted. I'm sure it doesn't choose a disk to be formatted... makes more sense for the user to choose.
In your first post you mentioned that you'd read the forums and some people say you can setup RAID1 without backing up the data and some people say you have to.
I'm pretty certain that the people who said you have to, also said why - in a nutshell - the DNS-323 is
known to have a "bug" which causes it to
occasionally format the "existing" disk rather than the newly installed one.
To answer your questions - what you think makes sense is of little relevance when you have no input into how the device works - however - since it is opensouce, you DO have the ability to change this and make it work the way you want - assuming that you have the relevant skills.
D-Link, for reasons best known to them, didn't see eye to eye with you and didn't give you the option to choose which drive to format - if it's any consolation - I agree with you, and I've suggested it to D-Link, but like your's, my opinion doesn't count.
@liketolearn - that's exactly how the DNS-323 is supposed to work, and when I have tested it, did in fact work, but feel free to search the forum and see the posts from those with different experiences.
@Bond13579 - your first post mentions not having the space to backup a terabyte of data - your second post says you understand what RAID1 does - so - with further ado, may I suggest that you'd be better off using the new drive for backup rather than redundancy.