Final answer:
A series of used clusters on a hard disk drive not associated with a file is called fragmentation, which can impact system performance. Defragmentation reorganizes data to reduce fragmentation and improve access efficiency.
Step-by-step explanation:
A series of used clusters on the hard disk drive that are not associated with a specific file is called fragmentation. When a file is deleted from the system, the data clusters that stored the file's data might not immediately be overwritten, resulting in these clusters becoming disassociated from active files. Over time, as new files are written to the drive, these fragments can lead to inefficiencies as the drive head must move to different areas of the disk to access a single file.
It's important for operating systems to manage a drive's storage space effectively. Defragmentation is the process used to reorganize the data on the drive so that files are stored in contiguous clusters, thus reducing the amount of disk seeking during file access and potentially enhancing overall system performance. Disk defragmentation utilities can be used to rearrange the file storage to minimize fragmentation.