I understood what was wrong: first sfdisk accept the size of the partition, not the increment, so the + sign is wrong. One difference from fdisk is that the end is the sector number from the beginning of the partition, not from the beginning of the device.

Then the unit cannot be other than sectors, so in my case, given the sector size of 512 bytes and a requested final size of aprox 7Gb , I had to launch the command as:

sudo sh -c 'echo " ,14596416" | sfdisk -N 2 /dev/loop14'

I understood what was wrong:

• First, sfdisk accept the size of the partition, not the increment, so the + sign is wrong. One difference from fdisk is that the end is the sector number from the beginning of the partition, not from the beginning of the device.
• Then the unit cannot be other than sectors.

So in my case, given the sector size of 512 bytes and a requested final size of approximately 7Gb , I had to launch the command as:

sudo sh -c 'echo " ,14596416" | sfdisk -N 2 /dev/loop14'

I understood what was wrong: first sfdisk accept the size of the partition, not the increment, so the + sign is wrong. Then the unit cannot be other than sectors, so in my case, given the sector size of 512 bytes and a requested final size of aprox 7Gb , I had to launch the command as:

sudo sh -c 'echo " ,14596416" | sfdisk -N 2 /dev/loop14'

I understood what was wrong: first sfdisk accept the size of the partition, not the increment, so the + sign is wrong. One difference from fdisk is that the end is the sector number from the beginning of the partition, not from the beginning of the device.

Then the unit cannot be other than sectors, so in my case, given the sector size of 512 bytes and a requested final size of aprox 7Gb , I had to launch the command as:

sudo sh -c 'echo " ,14596416" | sfdisk -N 2 /dev/loop14'