Workbench and the AmigaDOS Copy command and probably also DOpus4 use very small buffers and therefore do small reads on the drive.
DOpus5 is optimised for speed and uses large buffers so the read sizes can exceed the possible maximum transfer chunks on big files. Reducing MaxTransfer below this size will prevent the filesystem from reading more than possible in one go.
The possible MaxTransfer value depends on the controller and the drive. 64kb is small enough for all possible drives and controllers.
Bye,
Thomas