Apple - hdiutil convert/rsize resource temporarily unavailable

The problem appears to happen when you unmount the image using umount.

Remount the image (either using hdiutil attach or just double click the image) and use the command hdiutil detach it again. This will unmount and eject the image.

hdiutil detach /Volumes/<your image>

Another possibility is that the disk image is still mounted (from previous work on it). In this case, just select the eject icon.

Afterwards, the hdiutil convert worked great.

In my case something was trying to access the image. What finally worked:

sudo lsof | grep ~/Unix.sparsebundle/
sudo kill -9 889
resize -size 100g ~/Unix.sparsebundle/