Stress testing a hard drive with lots of random reads and writes
This seems to do the trick:

sudo fio --name=randrw --time_based --runtime=86400 --ioengine=posixaio --iodepth=64 --rw=randrw --bs=64k  --direct=1 --numjobs=8 --size=[size of disk] --filename=/dev/rdisk#

That should do a random read/write test for 24 The # can be obtained with diskutil list and [size of disk] is obtained with diskutil info /dev/disk#. I'm not sure if the --size argument is actually necessary but I included it anyway.