Problems mounting GPT partitioned external HDD

It seems that the drive has been formatted by Windows - which is not surprising, since Windows definitely must have been unable to use the disk which had very likely been formatted by OS X for sole use under OS X. Now the problem is exactly the same, just with the sides swapped.

If you want to mount the Windows partition, you can try blindly guess the file system:

mount -t FILESYSTEM -o ro /dev/sdd2 /mountpoint

where FILESYSTEM is likely to be (given the size of the partition) one of NTFS, exFAT or (less likely) VFAT. For NTFS one can use either in-kernel ntfs driver (in read-only mode) or the FUSE implementation ntfs-3g. exFAT has (allegedly) working FUSE implementation; VFAT has vfat. In any case consider doing the mount with -o ro or even creating a read-only loop device for the partition and mounting that. The reason for such a cumbersome approach is that some file system drivers may update the file system even if mounted in read-only mode (usually by fiddling with metadata). Which is definitely undesirable.

If you want to try to rescue the original (read "pre-Windows") data, check the Q&As referenced by Gilles (Recovering accidentally deleted files and How to recover data from a bad SD card?) and search the internet for file system recovery for the file system used by OS X, most probably HFS Plus.

As for the general question of "initialising a disk": I believe this happens whenever the system doesn't find a partition scheme it understands - this will happen either a MBR partition table or GPT on the disk - or if no partitions it recognizes are of "the right type". This can be surprising when one is used to Linux (and I would suppose BSDs as well) which doesn't pay attention to the partition types, instead caring about the actual content only.