Right now, there are some excellent hard drive deals at MacSales.com.
Hard drives slow down as they fill up, so even if capacity needs might fit into 6TB, an 8TB drive is a better long term investment. Ditto for 6TB vs 4TB, etc.
The MacSales 90-day replacement guarantee is something to consider when buying a new hard drive: waiting weeks for a hard drive manufacturer to send a refurb is an unpleasant plight.
OWC is proud to offer an extended replacement window of 90 days on new internal hard disk drives* (unless otherwise noted in the product description) and Pioneer DVR devices. Once a return authorization number has been issued and we receive the problematic drive back, you will get a brand new replacement drive, rather than a factory refurbished drive.
MPG also recommends buying drives with the OWC Thunderbay 4, to benefit from not only the full burn-in process but the excellent warranty.
Each ThunderBay undergoes OWC's multi-hour drive "burn-in" performance certification procedure prior to shipping. This ensures your ThunderBay arrives operating properly and ready for demanding use.
Back in late 2015 I invested in two OWC Thunderbay 4 units incorporating eight 8TB HGST 8TB Ultrastar He8 enterprise-grade hard drives, which cost a pretty penny. Those drives are still serving me well except for one failure, which was replaced by MacSales.