mergerfs!

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 fails.

StableBit DrivePool is a state of the art pooling solution.- Allows you to create a large pool with any number and any sized disks.- Files are stored on normal NTFS volumes, so you can recover your data on any system.- Drives are added in seconds, without having to format or forcing the disk to be used solely for the Pool.- Integration with StableBit Scanner to protect your data by moving it out of a failing disk.- Designed to be extremely light on system resources, so you won't even notice it's running.- Read Striping to boost performance from your pool.- Optional prioritization of network traffic (Network IO Boost) to make sure your network access is never slowed down.- Duplication is done in real time, so you'll never have issues with modifying files.- Complex and easy to use balancing system allows you extensive control over how files are placed on your disks.- Fully featured 30 day trial period.- Real time performance information (read, write, and read-striping). »

The driver combines a several mount points into the single one.

PoolHD is a disk pooling application that focuses on integration, speed and ease of use. Take two or more hard drives and create a storage drive that can be expanded seamlessly by simply adding more drives. PoolHD uses NTFS and does not interfere with a drives existing files system, meaning you can use drives for both pooled, and non-pooled use.