Cannot modify partition because of RAID

[Eric_Lee]

I'm installing 18.04 on a new machine and need to modify the partitioning of devices on which I installed software RAID. Per Want to Remove Software RAID (closed as solved) I have done this:
mdadm --stop /dev/md0
mdadm --stop /dev/md1
mdadm --zero-superblock /dev/sda1
mdadm --zero-superblock /dev/sda2
mdadm --zero-superblock /dev/sdb1
mdadm --zero-superblock /dev/sdb2

The installer no longer shows the RAID arrays but still sees the partitions as being RAID and won't allow them to be changed:

No modifications can be made to partition #1 of device SCSI (0,0,0) (sda) for the following reasons:
In use by software RAID device md0.

But /dev/md0 no longer exists.

How can I get back to where I can re-partition?

[TheFu]

Did you remove the RAID tag on the partitions?
Try

sudo dd if=/dev/zero of=$DEVICE bs=512 seek=$(( $(blockdev --getsz $DEVICE) - 1024 )) count=1024

where DEVICE is the partition, something like /dev/sda1

[lvm]

Eric_Lee, RAID device has to be stopped AND removed (--remove) prior to zeroing superblocks, otherwise it will come back.

TheFu, the command you mention does exactly the same as --zero-superblock.

[Eric_Lee]

Elsewhere it was suggested to use sudo fdisk /sda, then probably g for my environment. But I couldn't because the shell, ash, in the Installer doesn't have fdisk. I don't know if the same would have applied to dd.

Instead of Aborting the Install I unintentionally rebooted. No disaster. I started the Install again and when I got to the Partitioning it still had them marked raid (image) but there was no more No modifications allowed message when I tried to re-partition. Following these instructions the Install went smoothly.

Thank you TheFu and lvm. I ended up getting there a different way.

[TheFu]

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