As an experiment, I updated 10 m600 drives (2 are root in a mirror) on a Netflix machine serving about 8Gbps of traffic in parallel.
The box survived, but the drive terminated all the outstanding I/O when the drive restarted with the new firmware.
The normal retry / recovery mechanism rescheduled it them all that I/O and life went on with only a 3s hiccup while the drive was flashing the new firmware.
The 3s pause wasn't enough to cause any observable problems with the metrics we report, but maybe I was just lucky...