I think you're right. But just to be on the safe side, I did what somebody else suggested somewhere on this forum: get the first drive out, replace it by the blank one, format the blank one with the server's app, then switch it to the second bay and plug the first drive back in. The NAS had no problem with that and both drives work fine.
trouble is, I can't get both drives on a single aggregated RAID 0... but perfection is not of this world...
When plugging the NAS into the network by means of a CPL plug, I find myself back with my poor 8 Mb/s transfer rate. I guess the power line is just too old. I'll have to get back on a windows OS in order to put this to the test with the DLink app, of course.
That'll have to wait until Saturday, though.
Thanks for the tips.