Bilge

One thing to check while the floor is open, since you will not have this access again: whether anything else was cut for cables or ducts in the same joists.

It is common to find that whoever ran the plumbing was followed by somebody running wires who made their own holes in the same members. Two moderate holes in one joist are worse than one, and finding the second one after you have completed a repair is annoying.

Photograph everything with a tape measure in shot before you close it up.

Do not rely on the marking. There is a loose convention that the marked or striped conductor is negative, and it is honoured often enough to be tempting and violated often enough to destroy equipment.

I have shortened leads from a single batch of identical supplies and found the polarity swapped between units. Whoever assembles the cable is not always the person who decided which side the stripe goes on, and there is no standard that binds them.

The reliable sources of truth, in order:

1. The symbol printed on the supply. Almost every adapter has a small diagram showing a circle with a dot and a C shape, indicating whether the centre pin is positive or negative. That tells you what the device expects.
2. Measure it. A basic multimeter costs very little and is the correct tool.
3. Check the device's own label, which usually repeats the polarity requirement.

The symbol plus a measurement is two independent confirmations, which is what you want before connecting anything you care about.

There is a third reason that comes up in practice: you cannot verify the run is complete and correct once wires are in it.

A finished empty conduit can be checked — you can look through it, push a fish tape through it, confirm every joint is made up tight and that reaming was done. With conductors in place, none of that is possible, and the joints that were assembled last are the ones nobody looked into.

That matters most in metal systems, where the conduit itself may serve as the equipment grounding path. A joint that is mechanically together but not electrically continuous is invisible with wires in place and is exactly the sort of thing this rule prevents.