RAID 5 with four 4 TB drives: capacity, parity penalty, and rebuild-risk reality

How this is calculated

The parity calculation imposes a write penalty: every write requires reading the existing data and parity, computing new parity, and writing both. This makes RAID 5 significantly slower on random writes than RAID 0 or RAID 10. The bigger concern in 2026 is rebuild time. A 4 TB drive takes 8-24 hours to rebuild depending on controller speed and array activity. During that entire window, a second drive failure (more common than you'd think, especially with drives from the same batch) destroys the array. With 4 TB and larger drives, many storage professionals now recommend RAID 6 (double parity) for arrays of 4+ drives.

Verdict

The capacity math is 12 TB usable from 16 TB raw with single distributed parity, and read speed scales with the data drives (about 3x single-drive speed in this config). Write performance is the trade-off: every random write forces the controller to read the existing data and parity, recompute the new parity, and write both back. The architectural risk in 2026 is the rebuild window: replacing a 4 TB drive at moderate array load takes 8-24 hours, during which the array carries zero redundancy and a second drive failure destroys all data. Drives from the same manufacturing batch tend to fail together, which is the statistical reason the rebuild-risk window is real rather than theoretical. Push past 4 TB per drive or 4 drives total and RAID 6's double parity becomes the safer default, since it absorbs a second failure during rebuild at the cost of one more drive's capacity.