What are some alternatives?
When comparing Greyhole and StableBit DrivePool, you can also consider the following products
Drive Bender - Drive Bender, class leading storage pooling technology
mergerfs - mergerfs is a union filesystem geared towards simplifing storage and management of files across...
OpenMediaVault - OpenMediaVault is the next generation network attached storage (NAS) solution based on Debian Linux.
mhddfs - The driver combines a several mount points into the single one.
unRAID Server - A scalable, consumer-oriented, server operating system.
SnapRAID - A backup program for disk arrays. It stores parity information of your data and it recovers from up to six disk failures.