umount.udisks: 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.udisks - from package: udisks

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

On Debian possible default full paths are:

/sbin/umount.udisks - from package: udisks

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

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