mhddfs: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/bin/mhddfs - from package: mhddfs

You may install the required package with command apt-get install {package}

Error in other languages:
mhddfs: bevel nie gevind nie
mhddfs: comandă negăsită
mhddfs: comando não encontrado
mhddfs: commande introuvable
mhddfs: command not found
mhddfs: käsku ei ole
mhddfs: không tìm thấy lệnh
mhddfs: komanda nerasta
mhddfs: Kommando nicht gefunden.
mhddfs: kommandot finns inte
mhddfs: komut yok
mhddfs: nie znaleziono polecenia
mhddfs: níor aimsíodh an t-ordú
mhddfs: no se encontró la orden
mhddfs: no s'ha trobat l'ordre
mhddfs: opdracht niet gevonden
mhddfs: parancs nem található
mhddfs: perintah tidak ditemukan
mhddfs: príkaz nenájdený
mhddfs: příkaz nenalezen
mhddfs: команда не найдена
mhddfs: командата не е открита
mhddfs: 命令找不到
mhddfs: コマンドが見つかりません