After decades of working up through upgrades to MC I've been stalled in MC7.04. Now looking to upgrade and invest in a new system. (better late than never) I have always added more storage as needed with eSATA drives. I have an eSATA card on my HP Z400 with multiple ports and also a splitter through which I can access usp to 7 drives at a time. It's worked well for me. Is this the best option with a new system? I've also always run the MOJO DX on my system. Does MC 2020 still use this hardware? Finally, I ocasionally work with H265 4:2:2 from the GH5. Has the issue with this file format been solved in MC running Windows 10? Thanks in advance. 

Mojo DX:

https://avid.secure.force.com/pkb/servlet/fileField?retURL=%2Fpkb%2Farticles%2Fen_US%2Fcompatibility%2Fen422411%3Fpopup%3Dtrue%26retURL%3D%252Fpkb%252Farticles%252Fen_US%252FUser_Guide%252FNitris-DX-and-Mojo-DX-capabilities&entityId=ka15A000000tb8eQAA&field=Attachment_2__Body__s

Important not to run the latest Windows 10 version (2004) and stay with 1909.

Regarding the storage I recommend to get a solution that works best for your workflow. We don't know your requirments (speed, capacity, backup).

Certified Avid HP or other system is a secure way to go especially if editing is your main source of work - however you can always build your own system as well if you are comfortable with that.  I just built a i9 10920X - all works well.  Thee are also system builders out there as welll that will build a system for you.

I just bought a loaded, refurb HP Z840 for $2K and I added a Nvidia RTX2060 Super, (I'd go with a RTX30XX card now). RTX's aren't officially supported though. There's some Quadro cards out there if you want to be on an approved box. Check out Teksavers, 512-663-5764. The Z840 is a nice upgrade from Z400 or Z800s. They have a configuration tool on their site as if it were a new systems to be configured, which is cool for refurb machines, Warrantee is 3 years. And no, I don't work for them, just a very happy customer. 

HP Z Series Z840 Tower: 2x E5-2667 V3 Eight Core 3.2GHz Processors, 64GB (8 x 8GB) of RAM,1TB 6Gb/s SATA Solid State Drive, 1x Empty SSD Tray with Adapter, DVDRW, Windows 10 Pro Pre-Install, 1125W Power Supply.

They aren't system integrators but I sent them the Avid set-up guide for Z840 and they set it up based on that document without a charge. 

I added a BMD DeckLink 4K extreme 12G for $895. I have 3 Nitris boxes if you want one cheap, but I'd think you'd want something like the BMD. It's nice to be able to output Resolve and Premiere, along with MC output to the main monitor.

Good luck!