The message is actually saying that you will lose all the data on the drive that you have just ticked - not on those that were already ticked (and could not be changed).
When you click on that button with quite a few releases of the firmware you are taken to another screen that is in Japanese and displays a large 4-digit number for you to input to confirm that is OK. Enter the number and then click the left-most of the 2 buttons at the bottom to continue. When you do so, the system thinks for a minute or so while the RAID array is altered to put the new disk into it. During that period the raid array is not accessible via the network. You will then be told (hopefully) that the restructuring was successful.
At this point the system goes into RAID array "Repairing" mode as it rebuilds the data onto the new disk. This can take a long time - typically about 6+ hours on a 1TB system, and longer with larger arrays. The system is accessible via the network while the repair is in progress, but performance is degraded. When the repair process finally completes your system will be functioning with all drives once again operational.
I have been meaning for ages to produce a version of the telnet enabled firmware with the wording of those messages mage clearer and the one that displays in Japanese with one in English. The issue was that I knew how to live with the problem, and I had not got around to locating the text that would need changing. Maybe the fact that a couple of you have expressed confusion is a good reason to consider getting on and doing it