The reason for saying that is to restore the disks to 'as new' condition without any residual formatting to mess something up.
In the old days it was a case of using 'fdisk newmbr' today we use a rather sophisticated program that runs on most operating systems called DFSee available as a fully working evaluation copy at
https://hobbes.nmsu.edu/download.pub/os2/util/disk/dfsee165.zip it is a program that can do many things with disks and if one is not careful can also do nasty things to disks.
To use install one of your disks in a SATA/USB adapter and plug it into your computer, unzip the dfsee165.zip and select the dir for your OS, double click on the dfsxxx.exe (dfswin.exe for windows). Go to Edit on the top menu and select 'UI switch to advanced mode' in the drop down (this gives access to all the menu items). Then select 'Mode=FDISK' from the menu bar and move down to 'Partition-table operations >>' this will open an additional set of items, you need 'New MBR code, ERASE tables' again this will open extra items (all the disks the program sees).
MAKE SURE YOU SELECT THE CORRECT DISK BECAUSE ONCE YOU SELECT A DISK THE NEW MBR IS APPLIED TO THE SELECTED DISK.
Once you have the new MBR on the disk shut down DFSee and eject the disk. Repeat with the second disk then insert them into your DNS-323, power it on, make the RAID array plus the extra space. Restore the data from the backup and you are back in business.
NOTE If you are using DFSee for the first time take care,
lack of care can mess up your computer drives if you select the wrong disk. If in doubt ask.