Pi Remains Running When External (Powered) HD is connected

Question

I have a Pi3B+. Attached is a (powered) Western Digital My Book external hard drive. When I accidentally unplugged the power to the Pi, it kept running, happy as can be with the only other energy source being the the HD. My questions go along the lines of, but not limited ot:

• Is it Bad for the Pi?
• Is it bad for the HD?
• What is going on?
• It has not gotten very hot, but could it still be a fire hazard?

More Info:

• The Pi does not turn on when connecting the powered HD.
• I have not checked the logs, as I haven't done that since the update last year that spews 2.89 billion Undervoltage warnings/sec to the console.

Answer 1

TL;DR
The external hard drive is powering the RPI after you remove the microUSB power supply. This is called Backfeeding.
This is not unusual for raspberry pi's.

Is it bad for the pi

I would advise against backfeeding because there is little protection for the Pi. You would need to ensure there is a stable voltage and current to avoid damage. I would not continue backfeeding unless there is a good reason to do so.

is it bad for the HDD

It would depend if the USB power supply on the external hard drive was capable of safley delivering 2.5A (raspberry pi recommended power supply rating). T

WTH is going on?

The result of backfeeding might seem odd at first but simply the power is running from the USB Hub within the external hard drive back to the raspberry pi.

It hasn't gotten uber hot, but could it still be a Fire hazard?

All electronics are a fire hazard. I wouldn't have thought the RPI would ignite becuase it had 5V supplied from an external hard drive. I have not had a safety issue with accidental backfeeding. I would say you are more likely to have a broken RPI or hard drive than a fire.
If there was a power surge within the external hard drive USB hub, you could damage the raspberry pi.

The Pi does not turn on when connecting the powered HD.

The raspberry pi requires initial power from the micro usb port. Someone has provided the steps to backfeed

If you wanted to reduce the risk of damage to the raspberry pi, you could cut off the 5V+ supply from the HDD to the RPI as described in this article.

Answer 2

If the drive is supplying 5V power to the USB port, it will power the Pi. As this is unusual, and not complying with the USB spec, no one can say if it is safe.

NOTE You can not power a Pi through its USB ports, although once booted it will continue to run.