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Thanks for A2A.I’ll stick to HDDs, although the basic rules apply to Floppy Discs as well.Originally, Hard Discs were low-level formatted to write tracks and sectors but, otherwise contained no data. The the disc was ready for the Operating System to format it which would include “Look up tables” for the physical starting address of each file.It was common practice for the number of sectors (like slices of pie) to be the same for every track. (Some early Floppy systems used variable speed spindles so that there were fewer sectors in the center and more sectors on the outside, optimizing the amount of data that could be stored.)I won’t go into full file-system detail, as that is answer in itself which can be found on Wikipedia. When you want to read a file, the computer reads the “catalog” (Apple ][ talk) zone to find foldernames and filenames in the root directory, which will typically start at the same location on every disc. If the file is in a folder (or directory) then the OS has to read the folder from root level to find the address of where the directory listing is, which will typically be in the form of Head, Track, Sector. This address is fed to the drive controller which will move the heads to the selected Track, and wait for the selected Sector to pass under the selected Head. By repeating this several times, multiple layers of folder/directory can be traced until the physical location of the start of File is identified. If the OS has the capacity, much of this tracking information is stored in RAM to make it faster to locate a given file.This system was fairly consistent in the early days of Consumer Hard Drives which relied on a Controller Card and an analog driven Hard Drives. The first drives to appear on the consumer market which incorporated the Controller onto the Hard Drive directly (Integrated Disc Electronics) was the SCSI Hard Drive used by Apple Macintosh. When the parallel version came out was just called IDE. As drive capacities started to increase and certain factors could not be handled by the Operating System, Logical Block Addressing was introduced. This meant that the physical architecture of the HDD could be radically different from what the OS saw. If the Track-Count was too high, fake Heads enabled the OS to read the otherwise inaccessible tracks.Most modern HDDs are single platter with two heads, yet the geometry presented to the OS may be 16 (or more) heads. Also because the Controller is fully integrated with the mechanics, the Low-Level Format is done at the factory, and the number of Sectors on each track can vary, again, to optimize storage space, except that the OS never sees this. It still uses the physical addressing of a fixed sector system and the Controller sorts out the rest of it.As I’ve pointed out in other answers, the data density of HDD platters has crossed over the point of reliability and any data written will almost certainly contain errors when it is read back. Again, based on the older systems, the OS includes a certain amount of extra information for error correction purposes, but it is not adequate for the known level of faults on a new HDD. Therefore the Controller writes additional error-correction information so that it can read and correct before handing the data back to the Operating System.So when the Operating System wants to open the file “\documents\private\letterhead.doc”, it goes to the root directory listing starting at Head 0, Track 4, Sector 0 to find where the “documents” directory list starts, say Head 0, Track 56, Section 15. It then reads that “documents” directory list to find where “private” starts, say Head 0, Track 128, Sector 12. It then reads the “private” directory list to find where the file “letterhead.doc” begins, say Head 1, Track 42, Sector 19. It then reads that file into the application requesting it.Note: because tracking every single sector on a Hard Drive can occupy too much memory, Sectors are often grouped together in Clusters. The number of Sectors per Cluster is calculated to balance out the memory required for tracking versus the amount of unused space because of small files. This is why a short-cut on the desktop will occupy 1kB of disc-space even though the actual information may be way less.If you start with this Wikipedia article, you can read up on how different file systems (like FAT versus NTFS) handle the “header information” that enables an Operating System to find a particular file: File system