If you have a disk which can’t be mounted correctly (you plug it into your Mac but it doesn’t show up in Finder), this may indicate that the disk’s partition header is corrupted or the HFS+ catalog file was damaged. In this case, the disk will show up in Disk Drill and be shown as “Unallocated” or “Not Partitioned.”
Simply click the universal Recover/Rebuild button next to the disk name and Disk Drill will go through all of its partition find/rebuild modes. If the partitions can be recovered, they will be mounted as virtual partitions and automatically Quick Scanned. Your whole structure of existing files should be intact and can be recovered to a separate drive. The faulty disk should then be reformatted.
Alternatively, if it’s just the partition table that’s damaged, and Rebuild algorithm is not able to do its magic, it’s smart to try the new “Allocate existing data” recovery method, which produces very nice results being able to read what Mac/Finder can’t.