As a precautionary measure I would power down before seating the HDDs. My first word of advice would be to make a full backup of your data prior to performing any data/storage manipulation, but that's not an option here. Something you may want to try, and this is of course has its risks, is seating only one of the two HDDs and try powering up. The ShareCenter
may see the volume as a degraded RAID-1. Again, you try this at your own risk.
It's possible the Array was damaged by your old unit, as I also read your other thread. Data corruption in just the right place may make the volume unreadable by the ShareCenter, yet recoverable using other mounting techniques.
If all else fails, you may have a recovery option. If you can successfully mount and read one of the two HDDs in a Linux PC, then you can re-format the second HDD in slot-1 of the DNS-345 and simply copy your data from the PC to DNS-345. The risk here is that you're operating without a safety net since you'll be reformatting the only copy.
Once you get all this sorted out, I strongly recommend formatting the two HDDs as Standard Volumes and scheduling a backup job between the two. This approach will mitigate the risk of a failing RAID and will backup your data. RAID is NOT a backup, but rather only offers redundancy. Please see the following posts:
I should note that if you use Standard Volumes, the DNS-345 backup software can schedule nightly incremental backups, but does NOT have an option to remove files in the
backup that are deleted in the
source, so your backup volume will grow slightly larger than the source volume over time. I personally use a scheduled nightly backup and run backup software from a PC on my LAN once a month to synchronize the volumes (i.e. delete files in the backup that no longer exist in the source). I have 2 DNS-345 and 2 DNS-343, configured as 16 standard volumes. Working with so many volumes is far more cumbersome than working with a four RAID-5 arrays, but I will gladly trade data integrity over convenience.