According to Toller :
I have never used anything but grinding (what, blades, inserts, disks?) in
my Makita 4.5" angle grinder, but today I need a cutting disk. It is much
thinner, so it just spins. The retaining nut is flat on one side and
flanged on the other. If I turn it around so the flat side is down, rather
than the flanged side down, it seems to work okay.
But I don't have the instructions, so I don't know if this is actually
proper.
Any catastrophe happen if I use it like this? If so, how do I mount the
cutting disk?

In mine, it's the arbor washer that's "flanged" (it's actually a small
increase in thickness immediately adjacent to the hole - so I'll call
this the convex side).

The instructions say to use the convex side for thick wheels, and the flat
(actually, in my case it's slightly concave) side for thin wheels.

Check your grinder to see whether it seems more logical to flip the arbor
washer rather than the nut.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.