@Duske I can't be sure but I have two official Raspberry Pi 4 power supplies and experienced the problem on both. I also tried using my own SATA>USB adapter without any problem. Again, I can't be sure, but from what I tried, the most likely is that there is some 'kinky' stuff with this Retroflag adapter.

Isn't running everything from the SSD too much of a trouble if you have to start your Recalbox from scratch? How do you flash it from an img file?

@Adrian-Nawrocki Yes, like I said couple of hours ago, switching USB3 to USB2 solved the problem which is quite surprising though.

@Scavy Indeed. I worked after swapping USB2 and USB3: that's a bit weird. However, it seems just a little bit slower that when I use my own SATA>USB3. Thanks anyway, it will do the trick!

Nothing changes after I swapped those two USB. However, when using an SATA>USB converter on my own, it works like a charm whatever USB port I'm choosing. Seems to be an issue with my unit or with Retroflag