GL-AXT1800 max Micro SD card size

I know spec says 512GB but is there any reason 1TB wouldn’t work?

Short answer: Maybe the needed filesystem is not supported?

A little guessing: Most times because of the addressing limit.
We have block orientated filesystems in general, the blocks needs to be addressed and every address needs space.

For example: If you have 1MB and want a blocksize of 10kb, you’ll need a TOC (Table of content) of roundabout 102 ‘files’ (see inodes).
1MB=1024KB → /10 → 102,4 but an address can only be one, not a part of one.

But this means you can only store a maximum of 102 files, minus the TOC itself … The TOC need to contain every address and some meta information, like filename, size, ACL (or similar), date of creation, date of modification, …
Because you are clever, and don’t want to waste space on the TOC, you decide to store only 4 cluster with a size of 256kb each … Great Idea if you have 4 files with a size roundabout 250kb … But there is no space for a fifth file, even if all 4 files are only 4KB …

And don’t forget a directory is also a file (here is the reason for the ‘’ around file, above).
Wikipedia can explain this better than me: File system - Wikipedia

What I am trying to show: It’s complicated.

Sometimes it just don’t make sense, even if it would possible to use a larger storage, the access will be so slow, because the TOC needs to be accessed, cached and searched.
I don’t know what exactly is the reason for GL.iNet routers. But I trust the limitation they gave us.

I have a 1TB in mine. Works fine.

I do not see why to limit would be 512 GB.
If it were 2GB, 32GB, 2TB or 128TB I would say it would actually be a possible real limit.

In case of 2GB it supports only SD cards.
In case of 32GB it supports up to SDHC cards.
In case of 2TB it supports up to SDXC cards.
In case of 128TB it supports up to SDUC cards.

My guess is that it was tested up to a 512 GB microSD card, but would at least support all SDXC cards.