How are you doing the copy? Drag and drop or with some software? It could be a permissions issue; check (do a Get Info on the drives) and make sure that you have Read/Write permission).
On a Mac since 1984. Currently: 24" M1 iMac, M2 Pro Mac mini with 27" BenQ monitor, M2 Macbook Air, MacOS 14.x; iPhones, iPods (yes, still) and iPads.
