PerfectDisk's Boot Time (Offline) Defragmentation Strategy

The step-by-step process of Boot Time defragmentation is explained below:

When PerfectDisk starts an boot time defragmentation pass (scheduled or manual), it first tries to lock the drive without forcing the user to reboot. If the drive can be locked, the offline defragmentation pass can be performed, allowing the user to access other drives on the computer at the same time.

Note: Only drives other than system drive (where the operating system resides) can be locked for offline defragmentation while the computer is ON. System drives can only be defragmented during a system reboot.

If the drive cannot be locked, you are asked if you would like to force all open handles to be closed. If you click on Yes, then PerfectDisk attempts to close all open handles to that partition. If it is successful, then offline defragmentation pass is performed.
If the drive cannot be locked during a manual offline defragmentation pass, then you are asked to perform the pass during the next reboot.
During Scheduled offline passes, you are alerted prior to the scheduled start that the computer is going to be rebooted by PerfectDisk.
While the drive is locked, no other process can access the drive. If another process tries to update the drive, the operating system returns a message indicating that the drive does not exist.
The system drive, as well as any drive containing the Page File, cannot be locked while the operating system is running and therefore requires the system to be rebooted to perform the defragmentation pass.