I suggest you use Mackiloha for all GH2 modding. It includes arkhelper, which extracts and rebuilds many Harmonix ARK versions (and if you include the dtab executable in the Mackiloha folder (or in your path), it will also automatically convert DTB to DTA when extracting, and back when rebuilding. Mackiloha also includes superfreq, which is used for extracting and rebuilding the milo_ps2 files (which is where the majority of the textures are stored) with full transparency support (just make sure your PNGs are 8bpp for PS2 usage, it doesn't like 32bpp). These are both command line tools.
Also suggest you *ignore* any tutorials that suggest using GH Explorer, DTB Editor, or MiloExplorer. Those were once good tools, but nowadays are quite obsolete (the DTB editor is the only one with any use nowadays, being a way to find which DTB nodes are causing the game to die).
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