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.
1 In this example the first stair riser is the continuation of the base course of the retaining wall that the stairs are being built into.  Each stair riser will need a minimum of 6 in. (150 mm) of base material under it that extends a minimum of 6 in. (150 mm) behind the retaining wall block.