How do I mount a sparse disk image permanently?
The attach failed - no mountable file systems error seems to be a known bug of the -notremovable option to the hdid or hdiutil command respectively (see "hdituil -notremovable fails to mount disk image", https://discussions.apple.com/thread/2639671).
As a workaround, try adding your sparse disk image to System Preferences -> User Accounts -> Log in Items.
Yet another approach would be to write a launch daemon that mounts the sparse disk image at boot time (optionally for all users).
For a step-by-step guide in this respect see: http://wiki.ninjafocus.net/Mac_Mount_Points