Wouldn't it ship faster if you just plopped in any regular ol HDD the company makes (the cheapest one, probably) instead of developing a custom one just for that product?

If you're trying to bring a new drive to market (and even for many capacities, they get refreshed regularly if only to cut platters or find new ways to cut costs), and your firmware isn't ready for prime time as an internal drive, you can still ship it as an external drive earlier because you have a good idea of how the controller is going to treat the drive. Not only do the external drives tend to be the end of the waterfall in terms of quality, manufacturers also may ship with firmware problems that would be a stop ship if it was packaged for nearline or desktop.

Don't ask me how I know this.

And then consumers use external drives as a backup method...

Yes, if there's a take away for me, having seen how the sausage is made, it's far and above worth grabbing as NAS or nearline SATA drive and jamming it in a good SATA enclosure instead of buying a drive that's already in an enclosure if you want peace of mind.

Didn’t backblaze do exactly this (shuck drives from external enclosures) when they couldn’t buy enough drives on the open market after the tsunami?

And they themselves said that it's not a good idea, it simply was a time of desperation. They're not shucking externals now, now that proper drives are available.

Also they have a lot more tolerance for disk failure than almost anyone.

They did but with 3.5" drives which were not USB specific. This is done to save space and noone cares for space with a 3.5" drive. If you ship with a converter board it is impossible to make it this small.

I was always wondering how external drives can be cheaper than internal. This explains. Thank you.

Yeah, and massively cheaper too. I bought a 5TB passport drive for £71 recently, an equivalent capacity desktop drive would be at least £200. The price disparity is insane.

In reality, it is probably exactly that. These are the drives that didn't make the cut to be sold independently.

Yeah pretty common thing across all industries. I would expect things like maybe the vibration is slightly out of normal spec, or an abnormal amount of bad sectors on a fresh unit.

But I don't understand how them maybe not being top quality would make part of the SCSI protocol disappear in the firmware. That indicates that these drives had a different firmware developed for them with missing features which makes no sense.

What you are missing is that WD Passports are 2.5 inch drives that are USB-only. The drive controller speaks USB directly, as opposed to SATA chained to a translator chip on a separate board.

They implement certain parts not pass through and do indeed have separate USB firmware.