os-prober: 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 Debian possible default full paths are:

/usr/bin/os-prober - from package: os-prober

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

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