umount.hal: 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/umount.hal - from package: hal

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

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