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  • /dev/sda has linux system (SSD) (sda1-->mb fat) (sda2 linux ext4)
  • /dev/sdb1 is the tmp folder (ext4 SSD) (10gb unallocated for some
  • reason). All other disks have data and formated as ext4

Questions:

  • Why disks formated as ext4 file -s report that have entries "DOS/MBR boot sector"? what that means? Some times my linux system is confused and if i do not turn off these external disks, will not recognize the boot disk. How i can get rid of entries like "DOS/MBR boot sector" in all these disks? Note that gparted refer no label to be checked in all these disks.
  • Note that gnome-disks shows no unallocated space for all disks "sdc to sdo" with "1", BUT gparted for all these disks except sdc shows "1mb unallocated space". I know what is that. why "DOS/MBR boot sector" appears to only these disks that have "1mb unallocated space"?

i have read this about 1mb unallocated space. but no info about why dos mbr?

this is the output of file -s:

sudo file -s /dev/sd*
    /dev/sda:  DOS/MBR boot sector, extended partition table (last)
    /dev/sda1: DOS/MBR boot sector, code offset 0x58+2, OEM-ID "mkfs.fat", ......
    /dev/sda2: Linux rev 1.0 ext4 filesystem data,......volume name "linux" (needs journal recovery) (extents) (64bit) (large files) (huge files)
    /dev/sdb:  DOS/MBR boot sector MS-MBR Windows 7 english at offset 0x163 "Invalid partition table" at offset 0x17b "Error loading operating system" at offset 0x19a "Missing operating system"; partition 1 : ID=0xee, start-CHS (0x0,0,2), end-CHS (0x3ff,255,63), startsector 1, 234439534 sectors
    /dev/sdb1: Linux rev 1.0 ext4 filesystem data, ....
    /dev/sdb2: DOS/MBR boot sector, code offset 0x58+2, OEM-ID "mkfs.fat", sectors/cluster 16, Media descriptor 0xf8, sectors/track 63, heads 255, hidden sectors 214910976, sectors 19527669 (volumes > 32 MB), FAT (32 bit), sectors/FAT 9536, reserved 0x1, serial number 0xde973d4e, unlabeled
    /dev/sdc:  DOS/MBR boot sector; partition 1 : ID=0xee, start-CHS (0x0,0,2), end-CHS (0x3ff,255,63), startsector 1, 4294967295 sectors, extended partition table (last)
    /dev/sdc1: Linux rev 1.0 ext4 filesystem data,.....volume name "...." (needs journal recovery) (extents) (64bit) (large files) (huge files)
    /dev/sdd:  Linux rev 1.0 ext4 filesystem data,.....volume name "...." (needs journal recovery) (extents) (64bit) (large files) (huge files)
    /dev/sde:  Linux rev 1.0 ext4 filesystem data, ....volume name "...." (needs journal recovery) (extents) (64bit) (large files) (huge files)
    /dev/sdf:  Linux rev 1.0 ext4 filesystem data,......volume name "...." (needs journal recovery) (extents) (64bit) (large files) (huge files)
    /dev/sdg:  DOS/MBR boot sector; partition 1 : ID=0xee, start-CHS (0x0,0,2), end-CHS (0x3ff,255,63), startsector 1, 4294967295 sectors, extended partition table (last)
    /dev/sdg1: Linux rev 1.0 ext4 filesystem data, UUID=...., volume name "....." (needs journal recovery) (extents) (64bit) (large files) (huge files)
    /dev/sdh:  DOS/MBR boot sector; partition 1 : ID=0xee, start-CHS (0x0,0,2), end-CHS (0x3ff,255,63), startsector 1, 4294967295 sectors, extended partition table (last)
    /dev/sdh1: Linux rev 1.0 ext4 filesystem data, UUID=......, volume name "......" (needs journal recovery) (extents) (64bit) (large files) (huge files)
    /dev/sdi:  Linux rev 1.0 ext4 filesystem data, UUID=....., volume name "...." (needs journal recovery) (extents) (64bit) (large files) (huge files)
    /dev/sdj:  DOS/MBR boot sector; partition 1 : ID=0xee, start-CHS (0x0,0,2), end-CHS (0x3ff,255,63), startsector 1, 4294967295 sectors, extended partition table (last)
    /dev/sdj1: Linux rev 1.0 ext4 filesystem data, UUID=....., volume name "...." (needs journal recovery) (extents) (64bit) (large files) (huge files)
    /dev/sdk:  DOS/MBR boot sector; partition 1 : ID=0xee, start-CHS (0x0,0,2), end-CHS (0x3ff,255,63), startsector 1, 4294967295 sectors, extended partition table (last)
    /dev/sdk1: Linux rev 1.0 ext4 filesystem data, UUID=....., volume name "Books00" (needs journal recovery) (extents) (64bit) (large files) (huge files)
    /dev/sdl:  Linux rev 1.0 ext4 filesystem data, UUID=....., volume name "..." (extents) (64bit) (large files) (huge files)
    /dev/sdm:  Linux rev 1.0 ext4 filesystem data, UUID=....., volume name "...." (needs journal recovery) (extents) (64bit) (large files) (huge files)
    /dev/sdn:  DOS/MBR boot sector; partition 1 : ID=0xee, start-CHS (0x0,0,2), end-CHS (0x3ff,255,63), startsector 1, 4294967295 sectors, extended partition table (last)
    /dev/sdn1: Linux rev 1.0 ext4 filesystem data, UUID=......, volume name "...." (needs journal recovery) (extents) (64bit) (large files) (huge files)
    /dev/sdo:  DOS/MBR boot sector; partition 1 : ID=0xee, start-CHS (0x0,0,2), end-CHS (0x3ff,255,63), startsector 1, 3906963455 sectors, extended partition table (last)
    /dev/sdo1: Linux rev 1.0 ext4 filesystem data, UUID=....., volume name "...." (needs journal recovery) (extents) (64bit) (large files) (huge files)
Estatistics
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1 Answers1

1

why "DOS/MBR boot sector" appears to only these disks that have "1mb unallocated space"

In those cases, it's because the drive is partitioned (even if there's only one partition), and the first partition conventionally starts right after the first MiB (at LBA 2048, where LBA starts from 0, for drives with logical block size of 512 bytes).

Precisely with MBR/DOS-style partition table, the "post-MBR gap", namely LBA 1 to 2047 (hence 1 MiB minus 512 bytes), is not occupied by anything (except maybe embedded by the grub core image, which will not be seen as "allocated" by anything either way, if (BIOS / i386-pc) grub has been installed to that drive).

With GUID Partition Table (GPT), LBA 1 to 33 would be occupied by the actual partition table, with LBA 0 still occupied by e.g. a protective MBR. (It might be worth mentioning that with GPT, a backup partition table would be placed at the end of the drive, so precisely a GPT-partitioned drive would have slightly smaller usable capacity for its partition(s), even without considering the first MiB in the usual cases.)

You can also notice that in these cases, the ext4 filesystem is found at /dev/sdX1, which refers to the first partition of the respective drive, instead of /dev/sdX.

It might also be worth mentioning that, file would probably "resolve" any block device / file with its first block ending with the magic number (a hexadecimal sequence) "55 aa" to DOS/MBR boot sector, which is why you would also see that being show for e.g. a FAT-formatted partition.

Tom Yan
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