hdparm: 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:

/sbin/hdparm - from package: hdparm

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

On Debian possible default full paths are:

/sbin/hdparm - from package: hdparm
/usr/lib/debug/sbin/hdparm - from package: hdparm-dbg

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

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