Replace Seagate 3TB with Hitachi 3TB?

Should report the same size for the Hitachi. Remember 3TB in disk manufacturer's measurements is 3 * 1000 ^4 bytes. Now 3000 / 1024^3 * 1000^3 = 2794GB (approx). The NAS considers 1TB = 1024^4 Bytes. It's just a difference in measurements.

You could leave the 3 SeaGate drives in place and add the Hitachi drives as additional disks. If you just have three disks in the NAS you might wish to consider going to Volumes in Frontview and choosing the next disk you add to convert the array to dual-redundancy.

In my experience disk sizes tend to differ slightly between manufacturers. If the Hitachi drive is just 1 byte smaller than the Seagate drive it wouldn't work in a raid.

The only problem you would have is that you would have to back up, install at least one of the Hitachis and then do a factory default. But, if you need to do that, it would be better to install all the drives and then do the factory default.

Yes, sometimes you encounter slight volume variances between not only different manufacturers, but also between individual drives of the the exact make and model. I have 8 Seagate ST1000528AS drives in service between two ReadyNAS NVX units. At least three of the 8 are larger than at least 3 others. I encountered this when I lost one of the said drives in my main NAS and attempted to replace it with the exact same make and model I had as a spare (I had several spares as 4 drives, 2 in each NAS, had been upgraded to Hitachi 3TB HDS5C3030ALA630 drives). When I installed it, I got a rejection that the drive was too small. So I tried one of the other spares, and got the same result. Finally the last drive I tried was accepted and the array resynchronized.

I also encountered the same problem years ago with two Samsung 500GB drives trying to add them to an array on an NV+ with two Seagate 500GB drives. The first Samsung was added successfully but the second was rejected as too small. I wound up using a 750GB Seagate in bay 4 until some months later when all drives were upgraded.

So, even staying with the same make and model will not ensure you will not have a problem, although it is less likely. My only experience with mixing brands but the same size drives was the Seagate/Samsung episode and had a 50% success. With my Seagate ST1000528AS drives, it was 62% success. Almost 6 of one, half a dozen of the other. I will also admit these were all consumer grade drives, not enterprise grad drives.