Using voltage regulators for a computer harddrive
Let's check the current draw from the HDD documentation:



So it consumes 1.75A peak on the 12V line, and apparently doesn't even use 5V since there is no specification. That's high for a linear regulator. The linear regulator will dissipate (19-12)*1.75 = 12W peak. Since we don't know how long the peaks last and what "average" means, it is safer to design the heat sink for peak power. Then the 5°C/W spec on the regulator RthJC is a problem because the chip will be 5*12=60°C hotter than the heat sink, so it's a no-go. You need a buck converter.

19V 90W supply means 4.7 Amps. It'll probably be adequate, although you should measure the power drawn by the Nuc with GPU maxed too.

Unfortunately if you shop online with the usual suspects you will most likely find modules using low quality cheap fake components like "LM2596" ; quotes are a must since many of these modules do not have an actual LM2596, but instead an unknown chip with "LM2596" written on top. Also the last one I tested had the correct inductor value for a LM2596, which was too low for the counterfeit chip, so it saturated on every cycle. Also you will get garbage-tier general-purpose capacitors which can't handle the ripple current and run burning hot. Do not expect any kind of reliability from these abominations. A likely ending for that story is capacitor failure, followed by overvoltage on the output and death of whatever is connected to it.

So I can think about several options :

1) If the HDD does not require 5V.

Check if the Nuc can run reliably from 12V. If this is the case, replace 19V supply with a 12V supply. Done.

2) Use a buck to step down from 19V to 12V (and 5V). Here's one that should be reliable.


  this computer will be in an office 4+ hours away, I don't want to do that journey because one of the cables have "fallen out"


3)

In this case a much better option is to buy only commercial off the shelf parts that can be easily delivered if they break and replaced by the end user. Since a 12V power supply for an external hard drive enclosure costs less than a 4 hour trip, I recommend giving a backup one to the customer. If you're afraid of connectors getting unplugged, apply adhesive tape.