You should back up your system drive before you expand it. See the Microsoft® web page (www.microsoft.com) for more information about using the DISKPART utility.
Using the DISKPART command line utility on disks that have been changed from basic to dynamic in Windows may cause multiple partitions to be created for a volume which will cause performance degradation on iSCSI attached servers. To ensure the system drive is contained on a single partition, do not change the system drive from basic to dynamic. If using dynamic disks, specify sizes that will not cause the system drive to be misaligned as listed in the iSCSI Disk I/O section of System i™ Performance Capabilities Reference

To expand a system drive, do the following steps.

Shut down the server. See Start and stop an integrated server.
Unlink the system drive disk from the server. See Unlink integrated Windows server disk drives.
Change the size of the disk. See Expand a disk drive.
Link the disk to a temporary server network server description as a data disk. See Link a disk drive to an integrated server.
Start the temporary server. See Start and stop an integrated server.
On the temporary server Windows console, extend the partition of the disk using the DISKPART utility.
Shut down the temporary server. See Start and stop an integrated server.
Unlink the disk from the temporary server. See Unlink integrated Windows server disk drives.
Link the expanded disk to the original server as the system disk. See Link a disk drive to an integrated server.
Start the original server. See Start and stop an integrated server.