Hello nrosselli, The reason that dell states that you want to stop all I/O when doing firmware updates is that the unit will need to reboot during some updates.  Also there are times that both controllers will go offline at the same time causing your data to not be accessible for a few minutes.  We also state this to prevent data loss that can happen when updating firmware with host accessing the data.  what you want to do is to limit as much I/O that you can so that if the MD has to reboot to apply the update.  As you can do the firmware updates dell recommends that you have a tested backup before you begin.  Also for the disk once the firmware is applied the disk is going to reboot so any I/O on the drive will get stopped until the drive comes back online & that is where you can get corruption if all I/O is not stopped. Let us know if you have any other questions.
Dell recently released an physical disk firmware they are classifing as urgent.  Powervault documetation has a caution listed that all I/O should be stopped to the SAN prior to applying the updated firmware.  This means we would have to bring our entire enviornment down prior to doing the update.   Has anyone ever applied a firmware update to the physical disks without stopping all I/O traffic to the PV?
Physical Disk Firmware Upgrade MD322