Emphasis on _some_. This type of behaviour is more common on the compact 2.5" type external drives.
As of writing this comment, big external drives still come with 3.5" hard drives, with the only caveat being that some require you to disable the 3.3V rail on your SATA connector, either by cutting the wire or taping some pins on your HDD with kapton tape. Without the workaround some of these shucked drives will not power up properly when connected to a SATA power plug that also provides 3.3V.
I would like to see the measured power usage attached to those results. Depending on the laptop and its TDP the OEM has set, these results can vary quite a bit.
The browser does have this effect on Linux systems with pulseaudio. You will hear lots of static in your speakers, and after that the audio will be distorted until you restart pulseaudio.
What I cannot confirm is the relation of this to the speech-to-text engine. Reader mode most likely utilizes text to speech.
As of writing this comment, big external drives still come with 3.5" hard drives, with the only caveat being that some require you to disable the 3.3V rail on your SATA connector, either by cutting the wire or taping some pins on your HDD with kapton tape. Without the workaround some of these shucked drives will not power up properly when connected to a SATA power plug that also provides 3.3V.