Power vault working disk replace

Good morning, I have a PowerVault MD3200 with all the disk bays used and no spare-disk. I want to be sure (as I want to keep it as spare) if a brand new dell disk with the same capacity of the others will work and I want to check it with that SAN. I have one disk group with a three disk RAID 5 with data that is not important. I want to replace one of the three disks with the new one. I tried on the modular disk storage manager to press the "Replace" button but it's greyed out. Do I have to force Fail flag on one of the three disks to have the "Replace" button working? As the disk I want to replace it's perfectly working and I want to put it back on the array can the "Fail" flag be a problem or I just need to press the "Revive" button? What could be the right procedure in this situation?

Thank you

Matteo