The problem is that not many USB-C data ports do PD, and even when they do, they may not necessarily provide the voltage required by such disk.
And more importantly, what do available chips support? How easy is it to get chips that support PD + data?
This isn't my area of expertise, so I was hoping someone would be able to say -- Yes, it's possible, I do this all the time. Or -- it's theoretically possible, but not widely supported, so no one does it.
Two of the USB-C ports of my Dell XPS 13 9380 provide 5A/3A over PD, and the third one provides 5A/1.5A. All of them can negotiate both sink and source PD contracts.
If your laptop can be charged via USB-C port and you can attach peripherals to the same port, it's almost certain that it can negotiate a source PD contract as well. That doesn't automatically mean you'll get more than 5V though, so such PD port may still be useless for powering a disk drive enclosure without additional voltage regulation.
Many PD controllers support this for 'sourcing' (5V/1.5A or 5V/3A) natively, since it's a common use case for laptops.
Would this even be possible? I assume some devices can do PD both ways (for charging and as a source). But if that’s even possible, I’d imagine it to be rare. I’d love to know the case from someone who knows more.