Depends how you originally formatted the drive - Either in FAT32, NTFS or WBFS (proprietary format).
Since Windows can't mount it, I'm assuming WBFS (also assuming the hard drive isn't dead or malfunctioning).
Plug in your hard drive and when Windows prompts to format, press cancel. Do not press format otherwise you lose all data. The hard drive should be mounted to a drive letter but inaccessible.
Download Wii Backup Manager (from here - the official website is dead), extract and run the program, go to "Drive 1" or "Drive 2" (either or, it doesn't matter in this case), then select the "Inactive" drop-down box and select the drive letter your hard drive is mounted to.
The games should pop up in the listing. You can then select all games and use the "Transfer" drop-down box to export them in ISO or WBFS