I think I did a clean installation, this is my first Macbook. And I don't think there was a previous owner because it was new straight from the Apple Store.
I had the same issue during and after booting into Safe Mode unfortunately.
Here is what it put when I wrote
hdiutil attach ~/thefile.dmg -verbose
(I hope -verbose was the correct way of formatting but I'm not sure)
Initializing…
DIBackingStoreInstantiatorProbe: interface 0, score 100, CBSDBackingStore
DIBackingStoreInstantiatorProbe: interface 1, score -1000, CBundleBackingStore
DIBackingStoreInstantiatorProbe: interface 2, score -1000, CRAMBackingStore
DIBackingStoreInstantiatorProbe: interface 3, score -1000, CDevBackingStore
DIBackingStoreInstantiatorProbe: interface 4, score -1000, CCURLBackingStore
DIBackingStoreInstantiatorProbe: interface 5, score -1000, CVectoredBackingStore
DIBackingStoreInstantiatorProbe: interface 0, score 100, CBSDBackingStore
DIBackingStoreInstantiatorProbe: interface 1, score -1000, CBundleBackingStore
DIBackingStoreInstantiatorProbe: interface 2, score -1000, CRAMBackingStore
DIBackingStoreInstantiatorProbe: interface 3, score -1000, CDevBackingStore
DIBackingStoreInstantiatorProbe: interface 4, score -1000, CCURLBackingStore
DIBackingStoreInstantiatorProbe: interface 5, score -1000, CVectoredBackingStore
Attaching…
Error 35 (Resource temporarily unavailable).
Finishing…
DIHLDiskImageAttach() returned 35
hdiutil: attach failed - Resource temporarily unavailable