To extend a volume, 1. There must be some unallocated diskspace available right next the to exiting partition. If there is no diskspace or its not contiguous. Option will be grayedout. 2. Another reason is you maybe using old file system like FAT32. It can handle up to 32GB only. Consider using NTFS. 3. Last you maybe using MBR disk which has a limitation of 2 TB. Consider using GPT for bigger disk size.