Synology - Synology Hybrid RAID (SHR) allows you to mix and match drives of different sizes, as well as expand your volume later. Best of all, it is non-proprietary, so you can
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Synology Hybrid RAID Explained – Why it is AWESOME – NAS Compares
What is Synology Hybrid RAID (SHR)? - Synology Knowledge Center
What is Synology Hybrid RAID?
How to add a drive to a Synology Hybrid Raid (SHR) with data protection on DSM 7 - Next in Photography