HP A3550A - High Availability Disk Arrays Model 20 Storage Enclosure User Manual page 327

Disk Module Removal and Replacement
Caution 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.
Handle disk modules very carefully as they can be damaged easily by shock and
vibration.
Disk modules having different storage capacities or different model numbers should not
be bound into the same RAID group.
A3396A 4-GB disk modules should not be bound in the same LUN with A3666A LP
4-GB disk modules because performance can degrade.
Strap the ESD wrist strap to your wrist and ground it to the disk array chassis (or
suitable ground). Place the disk module on an ESD conductive sheet when it is
removed from its carton. Use ESD kit P/N 5182-4119 (supplied with each array).
Disk Module Removal and Replacement 7-3