I have completed my first upgrade from sata to nvme.
It replaced two 2TB Sata SSDs with an nvme 4TB in an OWC Envoy Express Thunderbolt enclosure.
I installed a 4TB Crucial P3 in the enclosure. I was aware that the copying (it was about 3TB) would take a little longer, but it ended up being a little faster than I thought. The P3 goes down in the write speed after about 1TB and even more after 2TB. But that's ok because you only copy that much data once.
Write and read speed via Thunderbolt ports of the Mac Studio and MBPro: 1500/1500, and that constantly. It is only half the speed because only 2 lanes are used, but is perfectly sufficient for me.
The temperature of the nvme never exceeded 40 degrees under full load. The case sits on top of the Mac Studio, which also dissipates the heat perfectly.
Tested with Opus and VSL, and tadaaa ... my Synchron I strings load super fast, OPUS flies ... exactly what I hoped for. The Synchron player also runs super smooth now. This was an excellent decision.
I will probably buy an Acasis case and another 4TB nvme later for the rest of the libraries, though the OWC gives me perfectly adequate speed. It only costs 85€, the Crucial P3 4TB 260€.
Thanks for your helpful tips and advice. This has given me so much pleasure today.
