hddtemp: 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 CentOS possible default full paths are:

/usr/bin/hddtemp - from package: hddtemp
/usr/sbin/hddtemp - from package: hddtemp

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/sbin/hddtemp - from package: hddtemp

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

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