At prpr's suggestion of
"You could always use dd to blank the disk (and then use the Humax software to partition and format) or run mke2fs manually to format whichever is the troublesome partition (presumably /dev/sda2)"
dd if=/dev/zero of=/dev/sda bs=1M
to blank the disk and every thing went fine after that.
Providing you have a recent version of the custom firmware then it doesn't matter if your laptop is disconnected see https://hummy.tv/forum/threads/fix-disk-continue-running-if-connection-dies.7656/#post-104254 Best advice would be to upgrade the custom firmware to the latest version before trying again because there have also been improvements to fix-disk.