HP A3550A - High Availability Disk Arrays Model 20 Storage Enclosure User Manual page 251

Caution Because the disk array incorporates redundant design features, it does not have to be
powered down while replacing disk modules, SPs, or BBUs. The fan pack and power
supply units are also hot-replaceable but removal and replacement must be completed
within two minutes or the disk array will report an overtemperature fault and shut down.
NEVER remove or replace faulted disk modules when array power is OFF as data can
be lost.
NEVER rearrange faulted disk modules when array power is either ON or OFF as data
can be lost.
About Replaceable Components 6-3