In my opinion (This is a personal comment) - if you are going to use HDD, you would necessarily need to increase the amount of memory and consequently the lru_mb cache. HDDs are very slow, the fastest of them with 15k RPM has 400 IOPS - And the most basic SSD has 5K IOPS and an NVMe has around 120K IOPS, up to 10 million IOPS read. In theory a DDR4 RAM can give you 1.7 million IOPS write. SSD, NVMe and RAM have in common low latency and fast access.
Realize? more memory resolves physical storage bottleneck problems.
When we are talking about Dgraph, this is a DB designed to use the maximum of SSDs or NVMe. If you use HDD you have to compensate for this. And compensate a lot because in this hypothesis you are tripling the work of the Dgraph. With less memory and greater work you will have problems as with any DB.
Even PG gets better with SSD see the chart.