A multi-stack optical data storage medium (30) for recording and reading using a focused radiation beam (40) entering through an entrance face (41) of the medium (30) is described. It has a first substrate (31a) with present on a side thereof a first recording stack (33) named L0, comprising a recordable type L0 recording layer (35), formed in a first L0 guide groove (38a, 38b). A first reflective layer (39) is present between the first L0 recording layer (35) and the first substrate (31a). A second substrate (31b) with present on a side thereof a second recording stack (32) named L1 is present at a position closer to the entrance face (41) than the L0 recording stack (33) and formed in a second L1 guide groove (37). A transparent spacer layer (36) is sandwiched between the recording stacks (32, 33). The first L0 guide groove (38a, 38b) has a depth GL0