Just to play back what the issue is:
1. You have a Mac and an external disk drive.
2. The external disk drive was originally formatted as FAT32
3. You stored your images on this external drive, and now they are not there.
Is this correct?
if so, can we get a bit more information:
1. You say the images are missing - are they flagged as missing in some catalog software (eg Lightroom), or are they physically not visible on the disk when you look in finder?
2. Are any files visible on the disk in finder?
3. Is the external disk even showing up in finder?
4. What can you remember doing prior to the images going missing - tidying up? running new software etc?
5. Have you plugged the external disk into another PC / Mac BEFORE the images when missing?
I would recommend you do NOT write anything to the external disk for the time being.