Sparse disk image error 'No space left on device' while limit not reached

[Robertico]

I created a sparse disk image of 400 GB with hdiutil (terminal) on a 500 GB external drive.
After I have attached the disk image, I copy data to it. After about 17 GB of data I get an error 'No space left on device'. Why is it that I cannot fully utilize the 400 GB and is it raising this error?

hdiutil create -size 400g -type SPARSE -fs APFS -volname test /Volumes/data/test.dmg
hdiutil attach /Volumes/data/test.dmg

Any help much appreciated!

 

[MacsBug]

I find it easier using Disk Utility to create disk images.

A few questions...
1. On your 500GB external drive how much free space is there?
2. When you do a Get Info on the test.dmg what is t he file size.
3. When you mount the disk image, and do a get info. How much free space is shown?

 

[Robertico]

Please find screenshots attached