My MacBook pro consistently takes hours to recognize a FAT32 (ms_dos) external hard drive. I have the same issue on a 2015 MacBook Air. The drive has 500G capacity and is connected via USB (no external power source.)
I did not have this problem on my old laptop, which ran Linux.
The drive contains 300GB of music and movies and 100GB of backups. It also contains a corpus of 13GB of 500B .doc file fragments (i.e. millions of files.) Could this be the problem? The file fragments were added sometime after I got my Mac. I can't recall if the problem started before or after.
Right now, I am executing
du -depth 1 to get an overview of the disk usage of each folder, but it takes ages to complete. I'm thinking this is another symptom of the same problem.