While upgrading this system from squeeze to wheezy, grub-setup was unable to read core.img to generate a blocklist (even trying multiple times). The system was left in an unbootable state, because the on-disk MBR was not compatible anymore with the newly installed core.img. After mounting the system from a live CD, grub-install kept failing, even if I just run grub-setup without creating a new core.img. I managed to make it work only after umounting and remounting the disk and manually running grub-setup. root@grml:/# /usr/sbin/grub-setup --verbose --force --directory=/boot/grub --device-map=/boot/grub/device.map /dev/vda /usr/sbin/grub-setup: info: Scanning for dmraid_nv RAID devices on disk hd0. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: scanning hd0 for LVM. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: no LVM signature found. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: Scanning for mdraid09 RAID devices on disk hd0. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: Scanning for mdraid1x RAID devices on disk hd0. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: Scanning for mdraid09 RAID devices on disk hd0. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: Scanning for mdraid09 RAID devices on disk hd0,msdos2. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: Scanning for mdraid09 RAID devices on disk hd0,msdos1. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: Scanning for mdraid1x RAID devices on disk hd0. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: Scanning for mdraid1x RAID devices on disk hd0,msdos2. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: Scanning for mdraid1x RAID devices on disk hd0,msdos1. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: scanning hd0 for LVM. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: no LVM signature found. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: scanning hd0,msdos2 for LVM. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: no LVM signature found. /usr/sbin/grub-setup: info: scanning hd0,msdos1 for LVM. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: no LVM signature found. /usr/sbin/grub-setup: info: transformed OS device `/dev/vda' into GRUB device `hd0'. /usr/sbin/grub-setup: info: /dev/vda2 starts from 4209030. /usr/sbin/grub-setup: info: opening the device hd0. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: Partition 0 starts from 1. /usr/sbin/grub-setup: info: Partition 1 starts from 4209030. /usr/sbin/grub-setup: info: guessed root device `/dev/vda2' and root_dev `hd0,msdos2' from dir `/boot/grub'. /usr/sbin/grub-setup: info: getting the size of /boot/grub/boot.img. /usr/sbin/grub-setup: info: reading /boot/grub/boot.img. /usr/sbin/grub-setup: info: getting the size of /boot/grub/boot.img. /usr/sbin/grub-setup: info: getting the size of /boot/grub/core.img. /usr/sbin/grub-setup: info: reading /boot/grub/core.img. /usr/sbin/grub-setup: info: getting the size of /boot/grub/core.img. /usr/sbin/grub-setup: info: root is `hd0,msdos2', dest is `hd0'. /usr/sbin/grub-setup: info: Opening root. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: Opening dest. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: setting the root device to `hd0,msdos2'. /usr/sbin/grub-setup: warn: This msdos-style partition label has no post-MBR gap; embedding won't be possible!. /usr/sbin/grub-setup: warn: Embedding is not possible. GRUB can only be installed in this setup by using blocklists. However, blocklists are UNRELIABLE and their use is discouraged.. /usr/sbin/grub-setup: info: will leave the core image on the filesystem. /usr/sbin/grub-setup: info: attempting to read the core image `/boot/grub/core.img' from GRUB. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: succeeded in opening the core image but the data is different. /usr/sbin/grub-setup: info: attempting to read the core image `/boot/grub/core.img' from GRUB again. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: succeeded in opening the core image but the data is different. /usr/sbin/grub-setup: info: attempting to read the core image `/boot/grub/core.img' from GRUB again. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: succeeded in opening the core image but the data is different. /usr/sbin/grub-setup: info: attempting to read the core image `/boot/grub/core.img' from GRUB again. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: succeeded in opening the core image but the data is different. /usr/sbin/grub-setup: info: attempting to read the core image `/boot/grub/core.img' from GRUB again. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: succeeded in opening the core image but the data is different. /usr/sbin/grub-setup: error: cannot read `/boot/grub/core.img' correctly. After remounting the file system: root@grml:/# /usr/sbin/grub-setup --verbose --force --directory=/boot/grub --device-map=/boot/grub/device.map /dev/vda /usr/sbin/grub-setup: info: Scanning for dmraid_nv RAID devices on disk hd0. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: scanning hd0 for LVM. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: no LVM signature found. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: Scanning for mdraid09 RAID devices on disk hd0. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: Scanning for mdraid1x RAID devices on disk hd0. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: Scanning for mdraid09 RAID devices on disk hd0. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: Scanning for mdraid09 RAID devices on disk hd0,msdos2. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: Scanning for mdraid09 RAID devices on disk hd0,msdos1. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: Scanning for mdraid1x RAID devices on disk hd0. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: Scanning for mdraid1x RAID devices on disk hd0,msdos2. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: Scanning for mdraid1x RAID devices on disk hd0,msdos1. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: scanning hd0 for LVM. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: no LVM signature found. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: scanning hd0,msdos2 for LVM. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: no LVM signature found. /usr/sbin/grub-setup: info: scanning hd0,msdos1 for LVM. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: no LVM signature found. /usr/sbin/grub-setup: info: transformed OS device `/dev/vda' into GRUB device `hd0'. /usr/sbin/grub-setup: info: /dev/vda2 starts from 4209030. /usr/sbin/grub-setup: info: opening the device hd0. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: Partition 0 starts from 1. /usr/sbin/grub-setup: info: Partition 1 starts from 4209030. /usr/sbin/grub-setup: info: guessed root device `/dev/vda2' and root_dev `hd0,msdos2' from dir `/boot/grub'. /usr/sbin/grub-setup: info: getting the size of /boot/grub/boot.img. /usr/sbin/grub-setup: info: reading /boot/grub/boot.img. /usr/sbin/grub-setup: info: getting the size of /boot/grub/boot.img. /usr/sbin/grub-setup: info: getting the size of /boot/grub/core.img. /usr/sbin/grub-setup: info: reading /boot/grub/core.img. /usr/sbin/grub-setup: info: getting the size of /boot/grub/core.img. /usr/sbin/grub-setup: info: root is `hd0,msdos2', dest is `hd0'. /usr/sbin/grub-setup: info: Opening root. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: Opening dest. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: setting the root device to `hd0,msdos2'. /usr/sbin/grub-setup: warn: This msdos-style partition label has no post-MBR gap; embedding won't be possible!. /usr/sbin/grub-setup: warn: Embedding is not possible. GRUB can only be installed in this setup by using blocklists. However, blocklists are UNRELIABLE and their use is discouraged.. /usr/sbin/grub-setup: info: will leave the core image on the filesystem. /usr/sbin/grub-setup: info: attempting to read the core image `/boot/grub/core.img' from GRUB. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: the size of hd0 is 19529728. /usr/sbin/grub-setup: info: the first sector is <4377950,0,512>. /usr/sbin/grub-setup: info: saving <4377951,0,512>. /usr/sbin/grub-setup: info: saving <4377952,0,512>. /usr/sbin/grub-setup: info: saving <4377953,0,512>. /usr/sbin/grub-setup: info: saving <4377954,0,512>. /usr/sbin/grub-setup: info: saving <4377955,0,512>. /usr/sbin/grub-setup: info: saving <4377956,0,512>. /usr/sbin/grub-setup: info: saving <4377957,0,512>. /usr/sbin/grub-setup: info: saving <4377958,0,512>. /usr/sbin/grub-setup: info: saving <4377959,0,512>. /usr/sbin/grub-setup: info: saving <4377960,0,512>. /usr/sbin/grub-setup: info: saving <4377961,0,512>. /usr/sbin/grub-setup: info: saving <4377962,0,512>. /usr/sbin/grub-setup: info: saving <4377963,0,512>. /usr/sbin/grub-setup: info: saving <4377964,0,512>. /usr/sbin/grub-setup: info: saving <4377965,0,512>. /usr/sbin/grub-setup: info: saving <4377966,0,512>. /usr/sbin/grub-setup: info: saving <4377967,0,512>. /usr/sbin/grub-setup: info: saving <4377968,0,512>. /usr/sbin/grub-setup: info: saving <4377969,0,512>. /usr/sbin/grub-setup: info: saving <4377970,0,512>. /usr/sbin/grub-setup: info: saving <4377971,0,512>. /usr/sbin/grub-setup: info: saving <4377972,0,512>. /usr/sbin/grub-setup: info: saving <4377973,0,512>. /usr/sbin/grub-setup: info: saving <4377974,0,512>. /usr/sbin/grub-setup: info: saving <4377975,0,512>. /usr/sbin/grub-setup: info: saving <4377976,0,512>. /usr/sbin/grub-setup: info: saving <4377977,0,512>. /usr/sbin/grub-setup: info: saving <4377978,0,512>. /usr/sbin/grub-setup: info: saving <4377979,0,512>. /usr/sbin/grub-setup: info: saving <4377980,0,512>. /usr/sbin/grub-setup: info: saving <4377981,0,512>. /usr/sbin/grub-setup: info: saving <4377982,0,512>. /usr/sbin/grub-setup: info: saving <4377983,0,512>. /usr/sbin/grub-setup: info: saving <4377984,0,512>. /usr/sbin/grub-setup: info: saving <4377985,0,512>. /usr/sbin/grub-setup: info: saving <4377986,0,512>. /usr/sbin/grub-setup: info: saving <4377987,0,512>. /usr/sbin/grub-setup: info: saving <4377988,0,512>. /usr/sbin/grub-setup: info: saving <4377989,0,512>. /usr/sbin/grub-setup: info: saving <4377990,0,512>. /usr/sbin/grub-setup: info: saving <4377991,0,512>. /usr/sbin/grub-setup: info: saving <4377992,0,512>. /usr/sbin/grub-setup: info: saving <4377993,0,512>. /usr/sbin/grub-setup: info: saving <4377994,0,512>. /usr/sbin/grub-setup: info: saving <4377995,0,512>. /usr/sbin/grub-setup: info: saving <4377996,0,512>. /usr/sbin/grub-setup: info: saving <4377997,0,512>. /usr/sbin/grub-setup: info: saving <4377998,0,512>. /usr/sbin/grub-setup: info: saving <4377999,0,512>. /usr/sbin/grub-setup: info: saving <4378000,0,512>. /usr/sbin/grub-setup: info: saving <4378001,0,267>. /usr/sbin/grub-setup: info: dos partition is -2, bsd partition is -2. /usr/sbin/grub-setup: info: opening the core image `/boot/grub/core.img'. /usr/sbin/grub-setup: info: writing 0x400 bytes.