FAT32 works for both OSes, but has a 4GB size limit per file, so it isn't ideal. However there is a 4GB file size limit. You could use NTFS but you will have to install additional software on the Mac in order to read AND write to the volume. exFAT then is the best option.
The riser duct is a problematic zone due to: -build-ups, alkali infiltration and chemical attack to the steel anchors. Following the general rule, we suggest lining the duct with bricks in the round sections and with castables in the square sections.
Galvanized Panning. Master Flow Galvanized Panning is used to seal the underside of the floor joist space to create a return duct. It is constructed using 30-gauge galvanized steel and can be cut to size to suit a variety of applications.1  30-gauge galvanized steel provides durability and sturdiness. 2  Seals the underside of the floor joist space to create a duct run to channel air back through the air return system. Seals the underside of the floor joist space to create a duct run to channel air back through the air return system. 2  16in. width is equal to the distance of joist spacing, which enables the panning to be nailed or screwed directly into the wood.