mhddfs is described as 'The driver combines a several mount points into the single one' and is a File Manager in the backup & sync category. There are six alternatives to mhddfs for Linux, BSD, FreeBSD and Windows. The best mhddfs alternative is OpenZFS, which is both free and Open Source. Other great apps like mhddfs are btrfs, StableBit DrivePool, mergerfs and Greyhole.
OpenZFS is an advanced file system and volume manager which was originally developed for Solaris and is now maintained by the OpenZFS community.
OpenZFS is the most popular Linux alternative to mhddfs.
OpenZFS is the most popular Open Source & free alternative to mhddfs.
Btrfs is a modern copy on write (CoW) filesystem for Linux aimed at implementing advanced features while also focusing on fault tolerance, repair and easy administration.
StableBit DrivePool is the most popular Windows alternative to mhddfs.
StableBit DrivePool is the most popular commercial alternative to mhddfs.
mergerfs is a union filesystem geared towards simplifing storage and management of files across numerous commodity storage devices. It is similar to mhddfs, unionfs, and aufs.
Greyhole is an 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...
Bcachefs is an advanced new filesystem for Linux, with an emphasis on reliability and robustness and the complete set of features one would expect from a modern filesystem.
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It does the same thing as mergerfs but with better flexibility and more features. It also more stable.