When you initiate a Bare Metal Recovery of the server, the operating system drive may not be located and the server recovery may fail.
On Extensible Firmware Interface (EFI) based systems, the hard drive order may change when booting from the recovery USB key. The USB backup drive may be detected prior to the operating system drive.
Disconnect any USB hard drives from the server and boot the server from the USB recovery key. When the server boots into the recovery environment, you can reconnect the USB hard drive.