umount.ewf: 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.ewf - from package: ewftools

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

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