Over at the community, someone posted Synology's instruction to them, on replacing a failed drive...
Basically, it says that to repair a degraded array, if your replacement drive is not the same brand as it replaces, it must be a larger size. This is the first I've ever heard such. I suppose that if you replaced Brand A with Brand B, of the same size, it could be possible that Brand B might be a sector smaller. But really? Does anyone do this?7- Boot up the NAS and verify that you can see your data, including iSCSI LUNs if you had any. The volume status at this point will be "Degraded".
8- (optional) Repair the volume under Storage Manager > Storage Pool > Repair and select Disk 1. It will need to erase this drive in order to add it back to the RAID.
Please note, if you want to buy a new hard drive for this purpose, it has to be 1) compatible with your unit AND 2)exactly the same brand and size OR 3)any brand with a higher size.