Perhaps you're thinking that the parity drive
would get busy and worn out with time.
XOR the new data with the parity data and write the result to the parity drive
The VideoRAID RTR320 and RTR320X systems are built around five-drive and 10-drive storage arrays, respectively, with one of every five drives functioning as a parity drive
to deliver extremely robust data protection without interrupting performance during drive rebuilds.
The tangible cost of eRAID is that an additional parity drive
equivalent is consumed for each incremental protection level.
Striping with a dedicated parity drive
(RAID level 3) combines high performance with fault tolerance.