To extend a dynamic spanned volume using diskpart, first you need to add the new disk to the system, then run the following:
C:\>diskpart Microsoft DiskPart version 10.0.14393.0 Copyright (C) 1999-2013 Microsoft Corporation. On computer: SERVERNAME DISKPART> list disk Disk ### Status Size Free Dyn Gpt -------- ------------- ------- ------- --- --- Disk 0 Online 60 GB 0 B * Disk 1 Online 4096 GB 0 B * * Disk 2 Online 4096 GB 0 B * * Disk 3 Online 4096 GB 0 B * * Disk 4 Online 4096 GB 0 B * * Disk 5 Online 4096 GB 0 B * * Disk 6 Online 4096 GB 0 B * * Disk 7 Online 4096 GB 0 B * * Disk 8 Online 4096 GB 0 B * * Disk 9 Online 4096 GB 0 B * * Disk 10 Online 4096 GB 0 B * * Disk 11 Online 4096 GB 0 B * * Disk 12 Online 4096 GB 0 B * * Disk 13 Online 4096 GB 0 B * * Disk 14 Offline 4096 GB 4096 GB Disk 15 Offline 4096 GB 4096 GB Disk 16 Offline 4096 GB 4096 GB DISKPART> select disk 14 Disk 14 is now the selected disk. DISKPART> online disk DiskPart successfully onlined the selected disk. DISKPART> attributes disk clear readonly Disk attributes cleared successfully. DISKPART> convert gpt DiskPart successfully converted the selected disk to GPT format. DISKPART> convert dynamic DiskPart successfully converted the selected disk to dynamic format. DISKPART> list volume Volume ### Ltr Label Fs Type Size Status Info ---------- --- ----------- ----- ---------- ------- --------- -------- Volume 0 H Veeam Offsi ReFS Spanned 11 TB Healthy Volume 1 G Veeam Offsi ReFS Spanned 39 TB Healthy Volume 2 D DVD-ROM 0 B No Media Volume 3 E DVD-ROM 0 B No Media Volume 4 C NTFS Partition 59 GB Healthy Boot Volume 5 Recovery NTFS Partition 450 MB Healthy Hidden Volume 6 FAT32 Partition 99 MB Healthy System DISKPART> select volume 0 Volume 0 is the selected volume. DISKPART> extend disk=14 DiskPart successfully extended the volume. DISKPART>