HP A3550A - High Availability Disk Arrays Model 20 Storage Enclosure User Manual page 214

SP Microcode Updates
Disk Module Firmware Updates
Caution Never reenable a peer SP while a disk module firmware update operation is in
progress. Doing so can interrupt the operation, causing permanent disk module and/or
data loss.
Exclusive access is required to download disk firmware.
All file systems should be deactivated while updating disk firmware.
Expert Tool
5-42 Support Tools Manager (STM)