One should test it. On internal raid it will drop faster in speed so if it fills up speed will drop.
I got around 300 mb with internal 4 x 1 tb drives. When you populate with 4 Hitachi enterprise grade 2 TB you should get 4 x 13x MB which is about 500 something, but also this is only my theory. Not tested in practice.
If it works then it is relatively cheap solution but only raid 0. Then you would need 8 tb of timemachine 🙂
Somebody could test it out. My new machine is still in factory still so i can not give any results yet.
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Margus
https://iconstudios.eu