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Just booted up my laptop after its shutdown after a sudden poweroff, caused by a discharged battery. I use Pop!_OS which comes with a disk encryption utility, which, in my case, has been always disabled by default (probably because I installed my distro as a dual-boot to Win8). But even though it was not enabled, in the middle of the booting process it showed me a message that said: "Please enter the passphrase for disk [diskname] (cryptswap)" even though I have disabled my swap partition by commenting it out in /etc/fstab months ago. Anyways as I don't have a passphrase, I pressed escape, which got me to the tty, and from there I pressed Ctrl+C, which let me completely boot up the system. After booting up, I ran dmesg and got this output:

[   26.489543] device-mapper: table: 253:0: crypt: Device lookup failed
[   26.489547] device-mapper: ioctl: error adding target to table
[   26.520538] Swap area shorter than signature indicates
[   26.700214] FAT-fs (sda8): Volume was not properly unmounted. Some data may be corrupt. Please run fsck.

My 8th partition is for the bootloader. I ran fsck on /dev/sda8 which said:

There are differences between boot sector and its backup.
This is mostly harmless. Differences: (offset:original/backup)
  65:01/00
1) Copy original to backup
2) Copy backup to original
3) No action

Could it be the cause of said disk encryption pop-up and, also, what are the other consequences that can happen after a sudden shutdown without a proper unmount of the disk partiotions?

2 Answers 2

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I believe the byte at offset 65 (0x41) is the "dirty bit" that is set when a drive is not unmounted properly. See this output I got:

fsck.fat 4.1 (2017-01-24)
0x41: Dirty bit is set. Fs was not properly unmounted and some data may be corrupt.
1) Remove dirty bit
2) No action
? 2

That was soon followed by the same prompt you got

There are differences between boot sector and its backup.
This is mostly harmless. Differences: (offset:original/backup)
  65:01/00
1) Copy original to backup
2) Copy backup to original
3) No action
? 

Not sure if the dirty bit triggers that behaviour, it would surprise me if it did as I imagine a bad unmount is something the OS boot would deal with gracefully. Thought if it's the boot partition...

Sorry this isn't a full answer, just came across this Q on a search for the same message and offset and thought I would share.

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Thank you @Xiao for your answer (https://unix.stackexchange.com/a/636761/73115) I had the same problem of the two messages:

$ sudo fsck.vfat /dev/sdb
fsck.fat 4.1 (2017-01-24)
0x41: Dirty bit is set. Fs was not properly unmounted and some data may be corrupt.
1) Remove dirty bit
2) No action
? 2
There are differences between boot sector and its backup.
This is mostly harmless. Differences: (offset:original/backup)
  65:01/00
1) Copy original to backup
2) Copy backup to original
3) No action
? 3
/dev/sdb: 724 files, 201834/1942740 clusters

after reading your answer I just hit 1 to remove the dirty bit and consequently also the second message was gone:

$ sudo fsck.vfat /dev/sdb
fsck.fat 4.1 (2017-01-24)
0x41: Dirty bit is set. Fs was not properly unmounted and some data may be corrupt.
1) Remove dirty bit
2) No action
? 1
Perform changes ? (y/n) y
/dev/sdb: 724 files, 201834/1942740 clusters

I tried this on two partitions with the same behavior. Both filesystems are correct now.

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