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Disclaimer: I am no Datto expert, so I could easily be wrong here.
That said, as far as I can tell, you have the model with the Gigabyte made motherboard, which does not have a SAS controller, so I assume you need SATA drives.
Regarding capacity, I would not be surprised if there is a software based limiter if you are using Datto's software, it seems that capacity is a key differentiator between different models, so it seems likely that you are somehow limited by that: https://www.datto.com/resource-downloads/SIRIS_3_Specs_US.pdf
Of course if you are using something like Truenas, there should be no such limit.
That said, as far as I can tell, you have the model with the Gigabyte made motherboard, which does not have a SAS controller, so I assume you need SATA drives.
Regarding capacity, I would not be surprised if there is a software based limiter if you are using Datto's software, it seems that capacity is a key differentiator between different models, so it seems likely that you are somehow limited by that: https://www.datto.com/resource-downloads/SIRIS_3_Specs_US.pdf
Of course if you are using something like Truenas, there should be no such limit.