How to transfer files from old drive(s) to new one
This is technically a software topic but, since you're only likely to use this when you get a new hard drive, I'm leaving it in the hardware section.
The first step is to go to Storage -> Disks and find out which device (/dev/sd#) your new drive was assigned.
Go to Storage -> S.M.A.R.T. -> Devices and Edit the drive to enable monitoring.
If you want to test the drive, see the previous page about testing used drives.
Go to Storage -> File Systems and either (triangle play button) mount the existing file system or (plus button) create a new one and mount it. EXT4 or BTRFS is preferred. BTRFS is mostly for bit rot protection but is completely redundant with snapraid.
Go to Storage -> Shared Folders and share the new drive by itself. Repeat this step for any old drives you wish to retire.
When you go to pick it under Shared Folders, don't just accept what OMV puts in for the path/directory. Instead, all the way to the right, there's a "folder tree" icon. Click that and browse to the root of where you want to copy the files from (should just be "/" to get everything on the drive, but if you are copying from a particular directory, it will be like "path/to/dir").
Find your way to the command line. The guide tells you how to use WeTTY. Login as root (you can use sudo -i to switch to root) and use the following command.
rsync -avP /path/to/dir1/ /path/to/dir2/ /path/to/destination/ - This example merges two sources to one destination.
-a (archive): Preserves symbolic links, permissions, ownership, and timestamps. Identical to -dR --preserve=all, preserving recursive structure, symlinks, and attributes.
-P: Combines --partial (keeps partially transferred files if the sync is interrupted) and --progress (shows a progress bar during the transfer).
-v (Verbose): Increases the information provided during the transfer, listing each file as it is processed.
v and P are optional. Use P, if you want to be able to resume large files. On resume attempts, use the --append or --append-verify switch to actually resume. See this page for further details on the command. https://docs.jbcoder.us/books/linux/page/useful-commands
Be sure to use the "Absolute Path" (it has a convenient copy button) from Storage -> File Systems to find the correct paths.
Once you've entered the correct command, you're likely to be in for a bit of a wait. Once it's done copying, you can add the new drive to your mergerfs pool or share it however you wish. Likewise, you can remove old drives from the pool, unshare, unmount and disconnect them. Be sure to remove all references before you disconnect any drives. This includes other services like File Browser. If you don't get an error when trying to unmount the file system, you most likely succeeded in removing any other references.