Anyone else have an issue with Zbrush3.1 crashing when using hot keys to save a tool? It happens to one of my users 3 out of 5 times using hot keys, when he doesn’t use the hot keys his tool saves fine.
If you want to save your tool – don´t use the Ctrl+S shortcut .
As far as I know it´s a known issue.
Use Tool -> Save As and I heard it´s even better to use SubTool Masters save feature -> Save ZTool.
( I believe I read somewhere that it´s possible to assign a Shortcut to Subool Masters save feature, too. But I can´t find the thread now and I never tried it — therefore I really can´t tell you if it´s true 
.)
Another possibility: Check the Preferences - Mem settings.
Or maybe your model has more Polys than your system is capable to handle.
Hope that helps.