Mount .NRG image file on OSX 10.6

You should parse the output of hdiutil, and mount the filesystems you're interested in - because you can't do what you're trying to do with mount.

The first few lines of the man page show supported syntax:

 mount [-adfruvw] [-t lfs | external_type]
 mount [-dfruvw] special | node
 mount [-dfruvw] [-o options] [-t lfs | external_type] special node

Notice for -a, there are no options to specify a node path, which makes your command invalid syntax.

(Also, I'd suggest testing with a proper image file - one that is an actual CD image. Your image is a hard-disk type, with an Apple Partition Map, and no non-HFS partitions. In Disk Utility, choose New -> Blank Image and note all the selections carefully. You want "CD/DVD"; you can then edit the partition map after you've created it; say, to change to two partitions.)

I eventually got the disk to mount whilst retaining compatibility with my bootloader. For anyone wondering, the compatibility conflict was in the FAT BPB (BIOS parameter block). Basically, the BPB has a field for specifying how many sectors compose a FAT cluster, where I manually supplied a value of 1. However, changing this to 4 fixed the issue. For more information on the BPB, you can see this reference page (you want to look at byte 13).

For anyone stumbling upon this answer randomly, this basically means that some older disks (such as floppy disks copied by dd) may not be able to be mounted by OS X Mountain Lion any longer. This is just a hypothesis based on observations of my own project, but hopefully this helps someone else running into this far-fetched issue.