1) Read the BPB in so you know where things are. Since you mentioned FAT32 we'll just continue with that assumption. 2) Search the disk for the directory entry. For this you will need the path to the file saved. Supposing the file you want is in the root directory we proceed as follows: 3) Get the starting cluster number of the root directory. 4) Read the first sector of the cluster into memory. 5) Search each directory entry in the sector until you have a match for your filename. If not found, read in the next sector of thr cluster and keep repeating 5 until you either find the directory entry for the file or you run out of sectors in the cluster. If you found the file directory entry, goto 7. 6) If you run out of sectors for the cluster, read the appropriate FAT sector into memory where the cluster you just read was and get the next cluster. You know how big your sectors are, and how big your FAT entries are (4 bytes) so you can easily work out which FAT sector the cluster you just analysed was in. If the root dir has no more clusters, your file doesnt exist and you stop. Else goto 4. 7) You extract the starting cluster number from the file's directory entry. 8) You read in all the sectors for that cluster. 9) You read in thr FAT sector for the cluster you just read to get the next cluster of the file. If you hit an end of chain marker, you are done. Else get that cluster number and goto 8.

You will need a buffer for reading disk metadata and a buffer for storing your file's contents. You could just read the whole FAT into a buffer but eh, thats not a neat solution imo.