One OS on sd card second Os on usb

[mojso]

is it possible to boot one os from sd card and second os from usb drive. not in same time

[procount]

Short answer 1: No, not using NOOBS by default.

Medium answer 2: If you only have 1 OS on your SD card and 1 OS on your usb drive, why not just insert the SD card to use the first OS, or remove it and plug the usb drive in for the second OS? No need to use NOOBS for that.

Long answer 3: Yes, if you understand how NOOBS works, how OSes are booted and are not afraid to get your hands dirty, you can modify it, but the boot partitions of each OS must reside on the same drive as NOOBS.

If you install NOOBS to an SD card and install OSes using it, you have the choice to install:

1. ALL OSes onto the same SD card, or
2. ALL OSes to a fitted USB drive.
   In the latter case, the boot partitions of the OSes are still stored on the SD card and only the rootfs
   partitions are stored on the usb drive.

If you install NOOBS to a USB drive and install OSes using it, all OSes will be stored on the USB drive (both boot and root partitions).

So to do what you want relatively easily, I suggest you first put NOOBS on the SD card and install both OSes to your SD card.
Now you need to create a partition on your USB drive and copy the whole of the 2nd rootfs from the SD card to the USB drive partition. Then you need to fix the references up so that the 2nd OS will use the rootfs on the USB drive instead of the one on the SD card. How you do this depends on the OS you are using. Typically, you have to modify the root= parameter in cmdline.txt on the boot partition, and the /etc/fstab file on the rootfs (on the USB drive).

If I haven't lost you already, you can probably Google how to do that. Have a look at the partition_setup.sh script for the OS you want to move to understand how NOOBS does it. You may have to learn about PARTUUIDS to do it properly, although if you only ever have 1 USB drive fitted, using /dev/sda1 might be adequate.