Just like in other Unix systems one can, for example, use fdisk or parted to manage hard disks' partition tables. After doing changes to a disk's partition table, the kernel has to be instructed to reinitialize its internal data structures: where does a partition begin, where does it end, etc.

With fdisk and friends this is done on Linux with the BLKRRPART IOCTL, which is used to tell the kernel to reread the disk's partition table.

parted also uses this interface on Linux, but for GNU Hurd, the corresponding function, libparted/arch/gnu.c (gnu_disk_commit), doesn't do anything at all. The infrastructure in GNU Mach is already there, linux/src/drivers/block/ide.c (ide_ioctl) <BLKRRPART> and linux/src/drivers/scsi/sd_ioctl.c (sd_ioctl) <BLKRRPART>, but the IOCTL needs to be routed from libparted through glibc's Hurd IOCTL interface, through Hurd's libstore, to GNU Mach.

This is not a huge project, and actually one that is suitable for someone who wants to start with hacking the system.