ExFAT is an updated version of FAT (Microsoft DOS's original File Allocation Table) meant for use across platforms (not only PCs) and on external drives. Don't confuse exFAT with EXtFS. ExFAT is still a Microsoft product, meaning companies (hardware companies, like memory card manufacturers, who want to use exFAT on their disks/drives) need to pay a licensing fee to Microsoft. It is not open source. ExFAT is the file system I recommend for external USB disks.
Fortunately...