It's not fool-proof, but it's also not fool-ish.
I would add one addtional step.
When you remove the hard disk, before you install the new disk and reformat, take the time to install the removed disk and verify that you have access to the data using your proposed methods. If perchance you don't, you can still return the disk to the DNS-323, which - because you have not powered it up - does not know that the disk was ever removed.
I have not done what you are trying to do with a linux system, but I have done somthing similar using Windows and the ext2ifs file system drivers.