Boot into NVMe drives from USB

Hi all, I’ve installed Debian in 2 (mdadm RAID 1 formtted with Btrfs) NVMe drives installed into 2 PCIe adapters. The motherboard doesn’t support booting from those drives, so I’ve installed Debian into a USB stick (and it works) and I wanted to add into grub the RAID NVMe drives.

os-prober doesn’t see the other Debian installation. fdisk -l shows the 2 nvme drives, but it doesn’t see md0:

Disk /dev/nvme0n1: 238.47 GiB, 256060514304 bytes, 500118192 sectors
Disk model: SAMSUNG MZVLQ256HBJD-00BH1
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 16384 bytes / 131072 bytes
Disklabel type: dos
Disk identifier: 0xab581c58

Device         Boot Start       End   Sectors   Size Id Type
/dev/nvme0n1p1       2048 500118191 500116144 238.5G fd Linux raid autodetect


Disk /dev/nvme1n1: 238.47 GiB, 256060514304 bytes, 500118192 sectors
Disk model: SAMSUNG MZVLQ256HBJD-00BH1
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 16384 bytes / 131072 bytes
Disklabel type: dos
Disk identifier: 0x863fc92a

Device         Boot Start       End   Sectors   Size Id Type
/dev/nvme1n1p1       2048 500118191 500116144 238.5G fd Linux raid autodetect


Disk /dev/sda: 14.32 GiB, 15376000000 bytes, 30031250 sectors
Disk model: Ultra Fit
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x1d46a293

Device     Boot    Start      End  Sectors  Size Id Type
/dev/sda1  *        2048 28028927 28026880 13.4G 83 Linux
/dev/sda2       28030974 30029823  1998850  976M  5 Extended
/dev/sda5       28030976 30029823  1998848  976M 82 Linux swap / Solaris

mdadm --detail --scan doesn’t output anything. How do I solve this problem?