EDIT:
Yes, you need to reboot (actually it would make sense to insert the microSD card with the router off and then boot/power it).
BECOMES:
No, the card can be inserted with the router powered up and it will be added /sensed automatically (though actually it would make sense to insert the microSD card with the router off and then boot/power it)
it’s sticking out too much, it’s more like 5mm.
you can see it the same instant in winbox , and afterwards you have to format it, but it shows up instantly as a removable device.
I thought that it would not have been auto-detected, good to know, though I still believe that inserting and removing cards is better done with the device off.
Correcting my previous post, just in case.