I'm using a Marvell-based QNAP TS-219 with Debian (currently Stretch) since many years with 2 x 1TB hard disk (both with DOS/MBR partition table) in RAID1: /dev/md0 mounted as /boot and /dev/md1 with LVM hosting / and /home.
Recently I bought 2 x 6TB hard disk, to upgrade the system. My aim is to use the two new disks instead of the old ones and migrate the system without reinstalling it. My first step was to swap one disk and exactly copy (gdisk) the partition layout from the old 1TB to the new 6TB disk and then add and synchronize the RAID1. All went well and the system boots even with only the new disk. During the second step - so swapping the second disk - I realized that... I need to use GPT partition tables and maybe something more (UEFI? Bios partition?) in order to use all 6TB and have a bootable system. BTW, the system uses U-boot. For now, I've just attempted to create a GPT partition table on the second 6TB disk and to create the two partitions for md0 and md1. The RAID1 works well as expected but, clearly, the system does not boot from this second disk alone.
What are the steps I should do in order to obtain a fully working system with the new 2x6TB disks, with GPT, without reinstalling? Unfortunately, I don't know much of how U-Boot works.
Thanks in advance for your help,