So, to conclude this topic I have managed to perform this task successfully, increasing the Storage Pool capacity from 1TB SHR RAID to 4TB using 2 x Synology 4TB HAT3300 HDDs using the instructions in this YT resource.
As the video is from 2020, it’s a little out of date as probably features DSM version 6 (mine is DSM 7.2) but it is still easy to follow if you follow the instructions on the screen as it progresses. The screen instructions are intuitive and worthy of the Synology brand.
This video gave me the confidence to tackle this task and it worked really well. It mentions consecutive 24 hour periods for the disk to repair but under DSM 7 the option for fast repair is now available and in my case it took about 1½ hours for the first replacement disk and 50 minutes for the second (the 1TB pool was ~60% to capacity).
I chose the Synology brand NAS’s in preference to the Seagate IronWolf because the Synology disks firmware can be updated automatically by the Package Manager behind the scenes.
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