Are you adding NetApp(part number X???A-R?) disks or Seagate disks which you have purchased as Seagate disks? Non-NetApp disks are not supported and will not work. NetApp disks has firmware peculiar to Data ONTAP, and are formatted for Data ONTAP.
This topic pops up from time to time and no doubt does on other vendors' forums as well. Disk drives are, "for the most part", just commodity items after all, right?
Unfortunately it is not quite so simple. NetApp, like other vendors, write into their control systems (Data Ontap in this case) checks to be sure that the hardware is of known type. Put in what isn't known, and you will trigger failures and/or shutdowns.
That said -- if you get a non-NetApp disk that happens to match the exact model that NetApp uses (not like they manufacture their own of course) there are ways to make the non-NetApp disk functional. I won't point you to any of them but with enough research and effort and sometimes extra hardware it can be done.
Using a non-NetApp disk that does not match a model that NetApp sells is an almost sure-fire exercise in futility, and your data will be at risk during the process.
Both options will void any support you have from NetApp. Again, not unique in the enterprise storage space.
If you really want to spend the time to figure out your specific case with specific disk you mentioned -- yes NetApp does/did have a 3TB SED model for their encryption solution, and I know (from general sources) that Seagate makes all of the NetApp SED's. I can't say, because I don't know, if the NetApp 3TB SED disk was the Seagate model you mentioned.
In my place, while I do understand and know how to use generically obtained disks in a NetApp system, unless I had an already unsupported sand box type system that I didn't really care about, I personally would not do it. If my management cared so little about their data as to condone an action like the one you propose to save a few bucks, I wouldn't put myself on the line as now being the one to permanently and solely support a system that is likely to fail in some odd way at the very wrong time. If your managers value their systems so little, I'd question the value they place on you as well -- both now and when future difficulties arise.