Hello,
I would like to install DietPi on a mini PC with Intel Celeron N5095 CPU. The PC comes with a pre-installed 256GB SATA SSD to which I would like to add a second 1TB SATA HDD. I would like to leave on the first disk (SSD 256GB) only the boot and root partitions (then maybe I also configure a file swap) and move home folder on a partition on the second disk (HDD 1TB), how should I proceed?
You can use dietpi-drive_manager to mount the additional drive and then create symlinks for the home directory. Or you mount the external drive over the home directory, but this would need to be done manually in /etc/fstab since dietpi-drive_manager allows to mount to /mnt only.
It actually takes alot to move a working directory to another drive
It’s doable but you have to use a live cd and not boot from the working installation
How to Move Home Directory to New Partition or Disk in Linux (tecmint.com)
10 easy steps to move directory to another partition RHEL/CentOS 7/8 | GoLinuxCloud
I have always wondered why on a fresh install we couldn’t tell it to install the /boot partition to the SD card, then have the OS installed on a MUCH faster and larger SSD or NVME drive…this way the SD card only is accessed/changed when an update to the kernel or anything that might change files on the /boot drive [less chance for corruption of the disk]
On RPi exactly this is possible with dietpi-drive_manager 
. Since other SBC images ship mostly with a single partition only, it is more complicated to achieve it there. But it’s somewhere on the roadmap to enable this as well. Requires to move the /boot content into the SD cards root, so that it can be mounted to /boot on the new rootfs 
.
But here it is about /home and/or /root if I understood correctly, which is trivial to achieve since no firmware/boot part depends on those.