Stellar Data Recovery Professional for Mac software supports FAT, HFS, ExFAT, and HFS+ file systems. If the software is not detecting the external drive, then the file system of the drive could be different.
Check the file system of your external drive. The file system of the destination drive (in this case, the external drive) must be FAT, HFS, HFS+ or ExFAT file system. Also, ensure that the external drive is connected properly to the system.
