Here's a tutorial on how to transfer gamesaves for modding them or replacing with new ones.

Required:
Modio (Beta)
Downloads/id=1285/modio-30-beta.html
USB Xtaf Xplorer
Downloads/id=713/usb-xtaf-xplorer-v17.html
If you have problems transferring with Modio (like seeing past Partition 0)
- Use the same instructions, except when transferring, use USB Xtaf Xplorer, not Modio.

A USB Flash Drive or your Xbox 360 Harddrive with a Transfer Cable.
The flash drive must be configured for use with the Xbox 360 first.
The gamesave you want to mod/replace must be moved to the configured flash drive by going to System Settings -> Memory. You may have to move the profile before moving the gamesave.

[b]Instructions (For USB Flash Drive, but similar for hard drive)[/b]
1 - Download your 100%/modded/whatever gamesave that you want and extract/save it to your desktop
2 - Plug your pre-configured USB Flash Drive into your computer.
3 - Open Modio.
4 - Click "Explore a Device".
5 - Click "File" -> "Open/Close Device".
6 - Expand "Content/-PROFILEID-/-TITLEID-/00000001/"
7 - Make a backup of the game save (if you have one) by drag-n-dropping or using "Extract" it to it's own folder on the desktop.

<ONE TIME ONLY>
You need to make an ID Set. What this does is save your Profile ID and Device (Xbox) ID. This is required so that when you go to load the gamesave, it matches with your Profile and it won't say the gamesave is corrupt.
1 - Open the gamesave you just saved to your desktop.
2 - Find and Click "Edit ID Sets".
3 - Rename "Unnamed Set" to whatever you want, EX.- Scott.
4 - Click "From File..." and choose the OLD gamesave that you backed up to your desktop.
5 - Click "Save"
</ONE TIME ONLY>

8 - Open the new gamesave in Modio and choose your ID Set then click "Rehash and Resign".
9 - Overwrite the old gamesave with the new gamesave by drag-n-dropping or using "Inserting File" and choosing the new game save (If you have a blinking LED indicator on your flash drive, watch it. Sometimes it takes a few seconds before Modio starts to transfer the file).
10 - Click "File" -> "Open/Close Device" to finish everything off.

Now just put the Flash Drive back into the Xbox 360 and copy the gamesave

Useful Information: Folder List

Content Location: /partition3/content/0000000000000000/-TITLEID-/-OFFERID-/
Save Location: /partition3/content/-PROFILEID-/-TITLEID-/-OFFERID-/
Music Collection: /partition3/mindex/media/ - The Music files are simply .wma with a custom header.

Content refers to Live Arcade Demos, Themes, Gamerpics, Videos.
Saves refer to the save data for each game.
Music refers to the music collection stored on the hard disk.

The PROFILEID is unique to every profile and is used to link saves to their owner. Saves are stored under each PROFILEID directory.

OFFERID's are basically a version number give to each piece of content or save. Developers can assign different version numbers to their content when it's distributed. OFFERID directories merely keep things organized.

The OFFERID's that always remain the same are the ones assigned to:
00020000 - Gamerpics
00030000 - Themes
000D0000 - Demos
00090000 - Videos
00004000 - Game Installs
00000002 - Extra Content
000B0000 - Title Updates

TITLEID's are basically another way of sorting folders. Each game has its own TITLEID. Here is a list of game TITLEID's.