I built a set of 27.5+ wheels for my wife's gravel touring bike. I tried to save a few bucks by using SLX hubs and rotors with the existing XT calipers. The goal is to be able to switch between the 29 X 2 and 27.5 X 3 inch wheels easily. My LBS assured me there was complete compatibility between XT and SLX drivetrain components. It turns out that the rotor is outboard about a half millimeter. It is not much, but the drag is noticeable. I could shave off a half millimeter from the inside of the rotor hub with emory cloth on a flat surface, but i'm afraid I could introduce a wobble. I could use spacers or modify the locknuts to offset the hub inboard. Does anyone have suggestions for a best practice for a solution for this problem. Thanks in advance.

On the disc side of the axle you could maybe put a 1/2mm thick washer between the locknut and the dropout or get a locknut on that's 1/2mm thicker.

When I've set up adjustment-free quick-change wheelsets on 6-bolt rotor hubs I've shimmed either the rotor or the locknut outboard depending on what's been needed.
Haven't seen any rotor shims for centerlock. Unless you can find some, your best bet is to shim the locknut outboard on the hub that has the rotor mounted wide.

DON'T try sanding the rotor down. It's unlikely to end well.
If nothing else, it'll take a lot of useful life out of the rotor.

Okay, sorry I was not more clear. I did not mean sand the rotor, just the splined sleeve the rotor is attached to that slides on the hub. Shaving off a half mm or so, as long as the locking ring seats, I thought it would be okay. Thanks for your suggestions.

Would need to do this with a lathe or mill. Hand sanding/filing won't be able to plain it straight.

I'd be interested in seeing if anyone has a solution for centerlocks.

Your only option likely is just recenter the caliper every time you switch with the two M5 bolts. It shouldn't take more than a couple minutes to realign it, although the big issue is pads wearing quicker since they have to rewear onto wheel swaps.

Getting hold of shims that can be used to set the distance between locknut outer face and rotor is fairly easy. So setting up two hubs to be similar enough is at worst a bit tedious.
If still within normal adjustability of the caliper, you're done.

It'd be possible to hand cut shims for the centerlock rotor too out of stock. But I've never seen any available for purchase.

I looked to see how much room I had to shave off based on where the locking ring would bottom out. I only had a half mm to take off and I determined that I would not bottom out the locking ring on the hub race if I was careful to take only that 1/2 mm. Bottoming out the ring would leave no torque on the rotor. I did not need too many passes over the flat table with emory cloth to take down that aluminum collar a half mm. Because the splines would help keep things in alignment, I was not concerned with precision within a few thousands. I almost took too much off the front though, because I only got a couple clicks of the locking ring before it was fully torqued. That did the trick. Thanks for everyones input.

Ideally you want to match the distance from rotor disc to bicycle central plane so wheels will interchange.

Rather than shave the hub to bring the rotor inward, consider shimming the other hub to bring out half a millimeter so they match.

Or, since the change is small, shim the end of the axle to push that hub inboard with reference to the dropout.

Either is fine, and both simpler than trying to shave the hub.

thinner/thicker axle spacer?

thicker on the disc end moves the whole hub over , thinner on the other keeps the overal width the same.

Or thicker, depending on which hub and which way you need to move that rotor