My N900's storage is getting rather full and i'm thinking of moving _everything_ to external storage, in this case a 256GB microSD card.

Question being: do large/SDXC cards work with the N900?

endsormeans has in his signature stuff about that for N8X0 devices.

I think I have used a 256gb in my N900 but since last year it is now in my pinephone to test multimedia.
In any case with the small screen I mostly use my 128gb sd for a large cache of audiobooks rather than video. There is still no replacement for Panucci and I still have not gotten it to run on Pinephone/Mobian.

I have a 128Gb SD card in my N900 - but had to partition it as 2 x 64Gb drives as this is the biggest the N900 seems to support reliably.

It did work as a 128Gb drive but sometimes failed to mount. Now with 2x64Gb for the last 2 years, never a problem to mount.

Funny, that you ask, because last week i was not shure, which microSD i should buy (used one with 200GB, net 183, is full): the 400GB Sandisk ultra fast with fastest read/write presented in photo- for 130 or the sandiisk ultra 512GB for 44 hangig between all other usb-sticks in computer-department.
To make it short, took the latter and after inserting, mkfs and 5min later: voila, 480GB are shown in df and are now filled near half with migrated stuff.
I need to say, that i'm taking frequently backups of SD, because i don't know, how much rear-mem is on card reserved for bad blocks.

Interesting. Said SD card I wanted is out of stock unfortunately so I haven't upgraded. Yet.

I recall now that when I first got my N900 I changed maybe /etc/fstab or another .conf file to let me use ext2 on my SD cards.
So you will need to do that.
I don't remember what I had to do but it is searchable on TMO.
I don't use Windows so it doesn't matter to me but ext2 will not mount on a windows machine, not sure about Mac.

The SD card I wanted in question (Samsung Evo+ 256GB) was in stock, after formatting it to FAT32 with Rufus I can confirm it works just fine in the N900.

I normally just stick it in the phone, sudo gainroot, partition and format (mkfs.vfat /dev/whatever). I partition it as I'm running flopswap, so have 2 swap and 1 data partition on it.

I think the big limiting factor is file size in fat32.