писать + куда или где

Let's raise the difficulty of our questions!

Куда
verb of motion + в/на + Винительный падеж
e.g. Он отправился в Москву

Где
simple verb (indicating position) + в/на + Предложный падеж
e.g. Он стоит в комнате

And now the question of 1 mil euro.

- I must certainly write this in my diary

Я наверняка должен написать это в ...
куда: мой дневник
где: моём дневнике

наверняка -- here: непременно, обязательно

I don't know any rule, but usually we write что-либо (observation) кому-либо (to collegue) где-либо (in a letter), а куда (to Paris) отправляем/посылаем the lines written.
So I would suggest: писать в дневнике, сохранять на компьютере, ...

Писать куда-либо means писать где-либо with an intention to send куда-либо.
https://www.youtube.com/watch?v=Nmy0HGq0iSY -- песня-шутка "Письмо в редакцию" она же "Дорогая передача!"
http://otblesk.com/vysotsky/c-kazt7.htm -- слова
http://worldelectricguitar.ru/Letter_to_the_Editor.php -- слова с подробной расшифровкой

Cunningly, в дневник, в журнал, etc are also common used variants. :dunno:
Probably you have to remember them as exceptions.

Quote:

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Originally Posted by maxmixiv

Cunningly, в дневник, в журнал, etc are also common used variants. :dunno:
Probably you have to remember them as exceptions.

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So my question was indeed difficult?

I bet it was.
Now that I have read my own post, I don't think I even answered the question.
My next effort: when в дневник, в журнал are being used, we want to convey the meaning of putting words into a container.
And the opposite case: normal writing is not to store words but rather to share, so a letter is not a container as such. Therefore, писать в письме, not писать в письмо.

Also, we can писать в стол meaning "to write and not give anyone to read"
https://ru-translate.dreamwidth.org/11676094.html

Based on similar thinking how I will translate:
1. We should upload in our website new material -> Нам следует разместить новый матириал в нашей веб-странице (Предложный падеж)
2. We should write in our website a more attractive introduction -> Нам приходится написать более привлекательное введение в нашей веб-странице (Предложный пажеж)
3. He is responsible for inserting data of our customers (e.g. names, phones etc) in our the CRM program -> Он отвествен за введение данных клиентом (т.е. имена, телефоны и т.д.) в нашей компютерной програме (Предложный падеж)

1 and 2) "в" is not applicable to web-site or web-page in these contextes. You must use "на".
I think it is because we observe it on display, and "in/inside display" is nonsense.
However, if you use "на" it is ok. Also we can really change grammatical case to:
Нам следует разместить новый материал на нашу веб-страницу.

3) Он ответственен за введение данных клиентов в нашей компьютерной программе.
Is ok.
Also can be:
Он ответственен за введение данных клиентов в нашу компьютерную программу.

As was said above there is little shift in meanings. A very thin one. It's hard for me to express it...
First one answers question "where inputing of data takes place? in our program (as workplace)." (где?)
Second one answers question "where data is placed to? into our program (as database)." (куда?)

Russian "где" and "куда" translates to english as "where", but in russian they stronlgy differs.