The start of a script for doing one long-test every. Will work fine if you have fewer drives than days in month.

test-drives.sh

#!/bin/sh

DRIVES=`/sbin/sysctl -n kern.disks`

DAY=`/bin/date +"%Oe"`

# yes, I could jump to the DAY-th element of the list
# Do you know how to do that?
TESTED=''
i=0;
for drive in ${DRIVES}
do
  i=`expr ${i} + 1`;
  if [ ${i} == ${DAY} ]
  then
    echo today I run a test on ${drive}
    TESTED=${drive}
    break;
  fi
done

if [ "${TESTED}XX" == "XX" ]
then
  echo nothing to test today
fi