well if you have read all you can and nothing seems to fit your issue...

you have tried and tested in the n900 card slot other different cards that you know work fine ...right??

and the cards you have tested were formatted properly ...right??

If testing different cards in the card slot results in the same result...
It could be something os related possibly...

But you have installed regular old fremantle and you have no issues with the os ....right??

I am imagine that there is some terminal command on your n900 you could do... to test the card slot

I have n900's and know enough to get by...
But ..
I specialize with n800's and n810's ...
my forte isn't the particulars of the n900.....purely general and basic concerning the n900...
So I cannot say that any specific prompt I could suggest would be successful....
Diablo and Fremantle aren't the same OS beast...

Better if someone here knows the n900 inside and out ....
and chimes in with answers in that regard...

Having exhausted all other avenues of possibility ...os and card ...wise ...
It may very well be a physical issue...
I had an n900 with a sd card slot issue that WAS a physical issue with the card slot...
The contact pins inside the sd card slot were not making contact...
So it is possible...
that your problem may be the contact pins inside the sd card slot itself...
The contact pins may be sitting too low to contact the card to read it...

you could try to VERY VERY gently lift the sd slot contact pins a little bit...
If they do not stay up..possibly taking a very thin piece of card and placing it underneath the raised pins may keep the contact pins elevated enough for a continuous connection..

It may enable the card to contact the pins properly so that a good connection is made to enable reading of the card...

Those are 2 good physical solutions to a issue related to the sd slot contact pins being the culprit...
But be careful and gentle..

If it is a physical problem...
and not os related or the card itself...
and it is indeed the card slot...
Then..
That is your next best option..

If it is the connection of the slot itself to the board...
That is a whole other issue...
and replacing the card slot with a new one...or at least repairing the connection between board and card slot may be necessary...

That is the best I have to suggest at this moment..
I may come up with a few other ideas possibly...later...
And..
I am sure others will weigh in on possible culprits and possible solutions...