I believe that your PI crashed due to deprivation of power.
I believe that your hard drive "sustained read" needed more power that your power supply could provide. The constant power depleted power in the output capacitors of the power supply. This doesn't happen in occasional reads where the output caps get time to replenish their charge.
You can test this using a USB power meter (picture below). When Raspberry Pi current draw increases, the cheap power adapter starts to dip voltage below 5V and then the Pi starts to stop working.
Raspberry Pi B+ has under-voltage indicator. Here is a helpful post.
EDIT (as explained to Chris):
All power supplies (including linear and swithers) have an output capacitor(s). This capacitor tries to store energy in case the there is a transient demand in output current. The cap charges to its full capacity after some time at power up. Upon usage, the power draw may increase marginally (fraction) than what the power supply can deliver. Therefore the energy in the output capacitor gradually decreases with time. The depletion is dependent on factors like:
- Capacity of the output capacitor. Transient capacity ∝ Charge carrying capacity or value of the capacitor in use. Measured in farad (F)
- Output capability of the regulator circuit.
- Sustained power draw from target.
When the energy in the cap is fully depleted, the power draw becomes higher than what the power supply circuit (excluding output capicator) can store and deliver. Hence the load does not get enough power (P=V.I). This is my understanding of the situation.
As I mentioned, a better analysis can be done by measuring current and voltage (output power of the power supply). OP has not provided any specifications about the power supply used.
I have personally seen these dropouts happen when using cheap 500 mA power supplies that are labelled as 1A. I would encourage OP to use a 2A power supply with the V, I monitor, and report back if issue still persists.