This is probably one of the most frequently asked questions here in the forum, so I thought I'd make a short guide to address it...
Chances are, you've lost the write-pin while you were changing the belt. It's a small metal pin that senses the position of the write-protection tab on the inserted disk and if it's missing all disks will seem to be write-protected to the computer. But before you go crawling around aimlessly on that patterned carpet, here's what you are actually looking for:
Yes, it's small. If you do find it, this is where it needs to go. It simply drops into the hole. If no disc is inserted at the time, it should go all the way in until the head is lying against the aluminium frame:
However, this might not be your problem. There are several versions of the Amstrad 3in drive and not all of them used a physical pin, some used optical sensors. So how do you tell if your drive had a pin in the first place? If your drives PCB has a black sensor like this one:
Or a plexiglass sensor like this one:
Then stop crawling around on your hands and knees like an idiot, your drive never had a pin, so you haven't lost it.
If however, your PCB has switch contacts that look like this:
Then keep looking, you really have lost it. If you still can't find it, then your only option is to make a new one. This is what you'll need to make:
This should be quite easy to make from a small nail, but you will need to ensure that the sides of the shaft are quite smooth so that it freely moves as intended.