When you open a Grid (wired, wireless, or portable), the mod scans the storage disks on that network. If remaining capacity for a resource channel is critically low, a WARNING! dialog appears instead of opening the grid right away.
You can ignore the warning once and continue into the grid, or close it and free space first.
From any Grid, use the warning triangle button to enable or disable capacity warnings per resource channel (items, fluids, and others). Choices are saved per player.