FAT File Allocation Table
The 1977 filesystem still sitting in every USB stick, SD card and EFI partition.
The design is blunt: a table with one entry per cluster, each entry pointing at the file's next cluster — a linked list across the disk. Simple to implement, which entirely explains its survival.
FAT32 caps files at 4 GB, as anyone who has tried to copy a film onto a USB stick has discovered. exFAT lifts the limit. No journaling, no permission model and notorious fragmentation — but the only format that everything reads.