Greyhole is described as 'Application that uses Samba to create a storage pool of all your available hard drives (whatever their size, however they're connected), and allows you to create redundant copies of the files you store, in order to prevent data loss when part of your hardware' and is an app. There are nine alternatives to Greyhole for Linux, Windows, Web-based, BSD and FreeBSD. The best Greyhole alternative is Unraid. It's not free, so if you're looking for a free alternative, you could try OpenZFS or btrfs. Other great apps like Greyhole are Whatoblock, StableBit DrivePool, mergerfs and EON ZFS Storage.