Learned this the hard way…
A little known rule of thumb about how much space you *really* have on a Macintosh hard disk (HFS+ format).
Leave 15% of your available disk space free!
Your 100 GB disk won’t function well (or worse) if you use more than 85 GB of it. Apparently it needs that 15% to manage files correctly; if it doesn’t have it, your data is at risk.