Spinpoint F4 HD204UI 2TB 5400 RPM working good so far

Granted you have a lot more NAS units than I do, From my count, you have 36 in service which if you don't have a good numbering system, can get confusing. And that does not count spares and drives in PC's. I currently track 38 3 1/2" SATA drives but have only two active NVX units and one NV+ that is currently out on loan to a friend who is experimenting with it. He's fostering the NV+ and may adopt it. I also have 2 or more drives in 3 PC's and several drives that are used to store information (they are no longer used for NAS service but hold system drive clones and NTSF backups. It's amazing how drives start to proliferate when you have multiple NAS units. I use small rectangular labels that I stick on on the drive and then write the number with a Sharpie. To me drive organization is as important as file organization.