You don't need to format it, it's already formated (FAT32 or NTFS are both compatible).
You only need to put the games in the correct folders, and renamed correctly.
• create a folder named wbfs at the root of the partition.
you_drive:/wbfs/
• if your games are in .wbfs:
- put the files in the /wbfs/ folder
- Launch the program WiiBackupManager, go to settings, FAT32 tab, select "Title [GameID]" and check the "rename naming convention on mount" checkbox.
- exit the setting, go to HDD1 tab, select your partition in the list. Done

(every games are renamed automatically when you mounted the partition)
• if you games are in .iso:
- Launch WiiBackupManager
- Launch the program WiiBackupManager, go to settings, FAT32 tab, select "Title [GameID]" and check the "rename naming convention on mount" checkbox.
- In the "File" tab, load the folder where you have your ISO files
- in the "HDD1" tab, load the partition where you have your /wbfs/ folder
- in the file tab, select the games (checkbox), and select "Transfer to HDD1"
It will convert/move/rename the ISO in WBFS files, taking a lot less spaces.
• connect the drive to the Wii and launch the loader.