How do you replace/upgrade hard drives in a window storage pool?

The storage management UI has options to add new drives to the pool and to remove existing ones. Storage Spaces remembers disks by their ID (stored within the disk itself), not by physical connection, so a disk can be moved or temporarily disconnected without damaging the pool.

The safest option is to first add a new disk to the pool, then prepare the old one for removal. Storage Spaces will "evacuate" data from the old disk, and the pool's redundancy requirements will be satisfied at all times. Once the transfer is complete, you will see an option to remove the physical disk entirely. (See official docs.)

(If you don't have enough SATA ports for both disks, you can temporarily connect one disk through a USB-SATA converter. As mentioned before, Storage Spaces won't care. Yes, the transfer will be slower but it's going to be temporary anyway.)

Meanwhile, disconnecting a drive while it still contains data and letting the new one rebuild from parity isn't a great plan; it means risking data loss if another disk happens to actually fail during the rebuild. (That's similar to traditional RAID rebuilds.)