I'd like to try TrueNAS, going to install and test it on my HP 54L. So how to migrate mirrored (SHD) drives into TrueNAS, is there any way, or I have to copy and reformat these drives?
yes, afaik. Synology is using mdadm raid with LVM and ext4 i think. TrueNAS is using ZFS. You can maybe mount it in SCALE to transfer the data but i think you want to re-purpose those disks? So i am afraid you have to store your data temporary on another storage.