> You are quite right, you can't rebuild a 4TB single disk in minutes. It's utterly impossible.
It is entirely possible and reasonable. There are 2 ingredients here:
0. Parity declustering. In a parity declustered array, e.g., 8+2 RAID6 can be used to stripe data across a large number of drives, say, 100 (rather than 10, as in standard RAID). This means, that only a small fraction (10% in this example) of each drive has to be read during rebuild. See Holland's thesis (http://www.pdl.cmu.edu/PDL-FTP/Declustering/Thesis.pdf) for details.
1. Distributed spare. By allocating spare space on each device, a fraction of *total* array bandwidth can be used for rebuild. That is, the wider is the array, the faster is the rebuild.
The funny thing is that this technology is 20 years old.