How to disable Xserve power saving while running ESXi
This isn't an answer as such but a fudge/hack that stops the timeout issue by writing regular random data to each drive (static data doesn't appear to work).
Create a shell script as below (mine is called .bmreadloop) on VMFS attached boot storage:
#!/bin/sh

while [ 1 -lt 2 ]
do

&lt;/dev/urandom sed 's/[^[:digit:]]\+//g' | head -c10 &gt; /vmfs/volumes/bonoboBoot/.blah

&lt;/dev/urandom sed 's/[^[:digit:]]\+//g' | head -c10 &gt; /vmfs/volumes/bonoboTwo/.blah

&lt;/dev/urandom sed 's/[^[:digit:]]\+//g' | head -c10 &gt; /vmfs/volumes/bonoboHDD/.blah

&lt;/dev/urandom sed 's/[^[:digit:]]\+//g' | head -c10 &gt; /vmfs/volumes/bonoboThree/.blah

sleep 10

done

Add run it on startup in /etc/rc.local.d/local.sh:
/vmfs/volumes/bonoboBoot/.bmreadloop &gt;&gt; /dev/null &amp;

It's ugly but it works!