DIR-857 manual says max 1TB. (I think I saw elsewhere that larger sizes work, but are not 'officially' supported. I don't remember if there were other caveats surrounding larger sizes.)
Manual also completely misses any discussion of setup / storage. {sigh}
Accompanying shareport plus manual speaks only to the windows accompanying access utility, not any router settings.
It would be UNREASONABLE for the behaviour described if a partition is actually present. Any device, seeing a partition with a filesystem it does not understand, should not touch it. But it SHOULD list it.
I can think of two possibilities where this might not be true - (1) GPT table instead of traditional FAT table; (2) whole disk used as single partition, with no partition table at all.
I can see (1) becoming more and more prevalent - it shouldn't usually kick in as an issue until you're using a disk greater than about 2.2 TB. Frequently even such disks can use a FAT partition table, with more than one primary partition defined - just nothing larger than about 2.2TB.
Which is all to say ... even if the 815 doesn't support NTFS, it should at least have shown you a partition was present. I would have hoped. Any other behaviour would be ... nasty / non-standard / nefarious / pick your word. Even if it shows a partition type of RAW.
-----
Actually ... I haven't gotten that far, but ... if you connect a device, are there options present for formatting it? Do you have a choice of format types? (I could but hope for ext4?)
That will probably answer the formats supported question.