Reconnecting to old volumes after swapping in old drives

If you did an "Export" before removing them, then all that is necessary is to re-insert them with power off and power on.  If you did not use the export function and they were not the primary volume, you can likely also just re-insert them with the power off. If they were the primary volume, then you need to remove all the drives and put in just the two old ones, then recover data to another device.  If they were not the primary, you should also be able to install just those drives and boot, then restore data to another device.

You may want to try these in read-only mode first, just to be sure it works and tne NAS doesn't try to reformat them, though it shouldn't when it sees a system partition on them.  Worst case, it would just indicate they are used drives.  If not in read-only mode, it will also install the OS that's currently on flash to the drives.