I get this quite often and no idea why it’s happening …
When doing anything and nothing specific, suddenly my cursor shows as busy and my external hdd starts-up, when it’s done so Zbrush quits for no reason. The error log is too complicated to spot anything out of the ordinary.
Anyone else get this type of problem ?
As I say when doing nothing specific, there is on pattern to when it happens.
You may have AutoSave enabled… I maxed the times in Preferences here as it might not choose a time appropriate for me. 
Not sure it is that, every time it auto saves seems fine.
Zbrush regularly does a couple things which will cause it to to access the disk and may give you a “busy” cursor. It will Auto-Save, if enabled, and it will compact memory to free up more virtual memory space. Those are both normal, and not set to activate unless the program has been idle for a set time.
Why you might be crashing and if it might be related to either of those, I couldn’t say.
Thanks for the replies. I can’t find how to turn off auto-save ?
You can only max out the times. It’s in Preferences. HTH