I´m not a Mac user but i believe this is caused because you have Zbrush installed in other partition rather than the OS is installed. Zbrush installs a VMem folder which is very heavy when working and can cause those problems. If that is your case may be some one here could tell you in your OS the location of the txt file that has the path for the Vmem.
In Windows it´s located in: C:\Users\Public\Documents\ZBrushData. there you can modify the ZBrushScratchDiskPath txt to any other location in your PC or Mac i guess
Hope this helps