You need to download this software:
WBFS Manager

Instructions
-Install using setup.
-Plug in the hard drive or USB stick.
-Run the application.
-Choose the correct drive letter.
-Click Load. (If you haven't already formatted the disk to WBFS, you can do that by clicking Format).
-You should now see any backups on the drive on the left hand side.
-You can drag and drop ISO files from Windows Explorer onto the right hand side or you can click the browse button.
-Click the Add to Drive button to copy them over to the WBFS drive.

 

(EDIT: this post was aimed at linuxguru)
*sigh*
Yes...that's one way.



However, if you dislike windows asking you to format your USB drive full of games every freaking time you plug the drive in your PC...then you can go with a FAT32 partition or a NTFS partition as well. In addition, you can use the drive for other things. Oh, and use already existing tools to defragment or even unformat your drive in case something would go wrong. If you go for FAT32, then you can use the drive for all your homebrew as well.


You'll use a different program, but the process isn't more difficult:
-download and install wii backup manager
-in the tab 'drive 1', choose the drive you would like to use. choose your already formatted usb drive here
-in the tab 'files', choose the iso you wish to add to your drive
-(optional) go to options - settings, click the FAT32/NTFS tab, and choose how you want the folder structure to look (I suggest 'store files in separate sub-folders', 'Title [GameID]' and '4 GB' for split size...but this is entirely cosmetical).
-after you added all your iso's to transfer, choose the 'transfer' button and then the name of your earlier selected drive
-if you added a lot of iso's, go for a cup of coffee...it may take a while, but it'll do everything automatically from here on out.
-once finished, just plug the usb drive in your wii and load up your favorite loader.


That optional part may need some extra explanation: USB loaders will recognize the games by their ID number. The modern ones (CFG, wiiflow, GX) have the convention that games must be placed in a \WBFS subfolder, and named in a certain way. Wii backup manager takes care of that for you, so as long as you don't go renaming the created folders and subfolders within that \wbfs folder, your games will be found without an issue.
(another thing wii backup manager does is get around the 4GB limitation of FAT32...if a game is larger, it'll split it up automatically. As such, larger games can be played with no issue whatsoever).