I found a solution provided thru this link:
It turns out that aside from having issues with fastboot commands, my vm is having a hard time writing to DBoard due to USB set-up as shown below
it should have been set to USB 2.0..... that fixed the writing time-outs
I followed the steps provided from link above (thanks to Nicolas )
I changed a few lines as highlighted below
// loop mount the raw img
mount -o loop -t ext4 rootfs_ext4.img ./mnt
// create a new rootfs image with the linux mkfs tools and with the content of the original image:
// the size can be small here, we will resize automatically on the first boot.
sudo mkfs.ext4 -L rootfs rootfs_ext4-new.img.raw 7G (here i used 7G as full size of my rootfs_ext4 based on df -Th)
sudo mount -o loop rootfs_ext4-new.img.raw rootfs2
cp -a mnt/* rootfs2
//create the fastboot sparse image
ext2simg -v rootfs_ext4-new.img.raw newrootfs.img
the newrootfs.img size is about 1.8G and i could write then boot to DBoard successfully.
... Also I tried to directly convert my rootfs_ext4.img (7GB) to sparse image using ext2simg and it also works.
I'm still trying to figure-out what exactly happen to steps provided by Nicolas, but if anyone could enlighten me that would also be very nice .... as far as i understand, they tried to shrink the original image into its actual usage size then sparsed it.