Ignore boot signature

[dbalsom]

    pub(crate) fn validate<E: IoError>(&self) -> Result<(), Error<E>> {
        if self.boot_sig != [0x55, 0xAA] {
            error!(
                "Invalid boot sector signature: expected [0x55, 0xAA] but got {:?}",
                self.boot_sig
            );
            return Err(Error::CorruptedFileSystem);

The absence of a boot signature does not mean there is not a valid filesystem on the disk.

[rafalh]

Official spec says this:

There is one other important note about Sector 0 of a FAT volume. If we consider the contents of the
sector as a byte array, it must be true that sector[510] equals 0x55, and sector[511] equals 0xAA.

Even if partition is not bootable it usually have a bootsector that prints an error. Have you found a non-conforming disk?

[dbalsom]

Yes there are many. Early PC/XT BIOS did not check, and subsequently many early (1980's) PC titles do not have the signature.

See: https://www.pagetable.com/?p=165

[rafalh]

As far as I know DOS 1.0 don't even have BPB in boot sector. I wasn't planning on supporting such old FAT implementations because it would add a lot of complexity...

[dbalsom]

DOS 1.0 specifically yes, but there are disks with BPB and no boot signature.

[dbalsom]

here is one example.

[dbalsom]

I can confirm that it rust-fatfs reads this fine if we disable the check, so I don't think its any extra work. I can understand if you don't want to deal with it if there is something nonstandard in the FAT itself.

[dbalsom]

my disk editor using a patched rust-fatfs to read this image: