Question: I have a Raspberry Pi 1 running Raspbian. I tried running the SD card from it on a Raspberry Pi 3 but it didn't boot.

I've read various things about upgrading a Raspberry Pi 2 install to run on Raspberry Pi 3, but little for upgrading from 1 to 3.

Can anyone please help?

Answer: 
What is the version of the OS you are running?

`uname -a`

If it is older than the introduction date of you R Pi2 then it probably will not work. The newer OSs are backward compatible. So I would make a new OS SD boot card.
**The OS for Raspberry Pi 1 is mainly the same as Pi 3,**
so you should be able to install the software for this transfer.

Put NOOBS on the Pi 1's SD and boot. Find the "edit config" area and change the OS to Pi3

First, find a USB to MicroSD card adapter (found on Amazon for dirt cheap). Plug in the Pi 3's SD Card and choose it as the copy to device. Choose the Pi 1's SD as the source, then begin the copy. This can take up to 30 minutes. Next, safely eject the Pi 3's SD Card and format the Pi 1's SD. Next, insert the SDCard into your Pi 3 and boot.

*You now have Raspbian Full on the Pi 3.*