Whenever you have a disk connected to your Mac, it can be accessed by other programs. iPhoto might try to access your camera, Time Machine might want to backup to an external hard drive, etc. This kind of access can overwrite your precious lost data and interfere with the recovery process.
Disk Drill has an effective solution to this problem. Before you begin your scan, click Extras next to the volume you wish to recover files from and select Remount Volume As Read Only. A padlock symbol will appear next to the volume name. This will prevent the disk from being accessed by other apps during the recovery process. This step is recommended in our How to Recover Lost Files with Disk Drill Basic tutorial.
Note that you can’t remount your Mac’s internal hard drive, but you can back up the entire drive or partition into a DMG (disk image). If you are recovering from an internal hard drive, be sure to follow our tutorial How to Recover Lost Files From Your Mac Internal Hard Drive.