The Downloads share is where the action occurs for nzbs, but Roon was writing to appdata, so this could be 2 separate issues i'm conflating.
I did a bit more digging, and I'm starting to think something is wrong with my cache array. I fired up a VM that was running Roon previously and realized it is showing out-of-space (domains share I believe) 99.8% used of 8GB. I also got Roon running on Docker, pointing to a share that only runs on the array, no cache, and it seems to be working fine now.
I did follow this article, however, neither drive shows full and the rebalance didn't help. I did plan on adding more space, maybe my old 250GB SSDs aren't sufficient, even though it seems like they should be?
Label: none uuid: 169dda69-6f92-4350-b51a-d9c7d5b9fbdf
Total devices 2 FS bytes used 45.48GiB
devid 1 size 232.89GiB used 73.03GiB path /dev/sdf1
devid 2 size 232.89GiB used 73.03GiB path /dev/sde1
Data, RAID1: total=72.00GiB, used=45.36GiB
System, RAID1: total=32.00MiB, used=16.00KiB
Metadata, RAID1: total=1.00GiB, used=119.44MiB
GlobalReserve, single: total=39.62MiB, used=0.00B