I'm having problems creating new shapefiles - in QGIS 2.14.20 any new fields I create and any associated data are not saved. I've now got the same problem in QGIS 3.0, but there I get a message saying layer creation failed. There is an error message but it just seems to say 'warning :' But under OGR I get:
2018-05-10T10:44:30 WARNING Creation of OGR data source test.shp failed: Failed to create file test.shp: Permission denied
I am running on an iMac High Sierra 10.13.4. I also sometimes can't save projects. I am the only user of this machine and have admin privileges, and am trying to save the shapefiles and projects locally.