As part of the latest system software update, the PlayStation 5 can now support internal SSD storage expansion. Currently only available if you've signed up as a firmware beta tester, that came with a warning that not all games will automatically perform as well as they do on the PS5's internal SSD.
Now, Insomniac Games has carried out their own internal tests, using PS5 exclusive Ratchet & Clank: Rift Apart to see how well an external M.2 SSD's performance works for it. According to Insomniac's director of Core Technology, Mike Fitzgerald, there's good news.
Insomniac Games Confirm Ratchet & Clank Performance On M.2 SSDs Is "Almost Indistinguishable" From Internal PS5 Memory
Though he acknowledges these are early results, Fitzgerald advised the performance is looking good. He doesn't specify what specific Gen4 drives were used, though he confirmed the SSDs used the recommended specs, giving results that were "almost indistinguishable" from the PS5's internal SSD.
However, Fitzgerald confirms that for those using below-spec Gen4 M.2 drives, Insomniac saw "up to 15% slower loading in the most SSD-stressing areas" of Rift Apart. As such, he advises keeping an eye on the tech specs of your chosen model. Right now, only Seagate's FireCuda 530 and Western Digital's WD_BLACK SN850 are officially confirmed as PS5-ready.