Earlier today, I was using iotop to check the disk activity for the processes on my machine, and immediately started seeing the following error (or variations on this error):

dtrace: error on enabled probe ID 5 (ID 992: io:mach_kernel:buf_strategy:start): invalid user access in action #3 at DIF offset 0

I thought my SSD was dying (it’s something I fear!) – however, after some messing around I discovered iotop doesn’t work when iTunes is running, because Apple have disabled dtrace while iTunes is running. Quitting iTunes sorted the problem for me. There is more about this “quirk” here: http://www.securitypronews.com/insiderreports/insider/spn-49-20080124AppleAlteredDTraceToolSaysLeventhal.html