SOLUTION: When OSX won't recognise an exFAT external hard drive

A little gotcha with exFAT (a disk format like FAT32/NTFS etc which both OSX and Windows support out of the box) is that silly old OSX will only recognise the disk if the cluster size is 4,096 or less.

So if you can't see an external exFAT disk from a Mac, you probably need to backup the data and then reformat the disk with a cluster size of 4,096 or less.. if that doesn't work, carefully delete the partition from OSX and recreate it.

4,096 is rather shit for large files like photos, mp3, etc but anyway, it works