Also, please remember it is not the controller doing a drive firmware upgrade. The drive itself has to do a reset to apply the firwmare on some types of disk drives. For example, most SATA HDDs need to do a power cycle before the drive firwmare gets applied. Depending on your raid version you would not want host IO being applied to the drive while doing a reset. Having no host IO to the disc helps protect the raid set and user data.
Cheers,
Shawn