To recover unallocated hard drive space on Windows, you can start by using the built-in Disk Management tool. First, right-click on the Start menu and select "Disk Management." Locate the unallocated space, right-click on it, and choose "New Simple Volume" to create a new partition. Follow the prompts to assign a drive letter and format the new partition.

If the manual method doesn't meet your needs or if you're dealing with more complex scenarios, using a dedicated recovery tool like BLR Data Recovery Tool can be beneficial. This tool offers advanced features to scan for and recover lost partitions, as well as to restore data from unallocated space. Simply launch the tool, select the drive, and initiate a scan to recover the unallocated space and restore any lost data effectively.