What is the size of your other disks?  What is the size of your replacement disk?  What are the makes/models of all the disks.  It is normal for every make and model to vary in size slightly, so if your replacement is even a few MB's smaller than the one it is replacing, then it will be unsuitable as a hot-spare. No, formatting is possible, but not necessary.
We welcome your help. We have a legacy system running on a Dell Poweredge 4400 with Perc 3/Di with a RAID 5 virtual disk (Windows Server 2003) . An Array Disk was recently downgraded and automatically replaced by a dedicated hot spare.  We powered down and removed the degraded array disk and replaced with a new disk the same size as the others in the array. Ssytem rebooted without problem.  Using Dell Open Manager Array Manager (3.7), we were able to initialize the new array disk, run a full diagnostic on the new disk and it is now listed as "Ready" in Dell Array Manager.  When I try to configure "add dedicated hot spare", the new array disk does not show up as available in the hot spare wizard.   When I right click on the new disk, it gives the option of  "assign as global spare"  but when I select the option, I get an error message saying disk is too small.   (The server has never had a global spare before) The new disk is also does not appear to be included as an active  part of the virtual disk. The new status is "ready,"  but the controller is not doing anything with the new disk. My question is how to I get the controller to make the new array disk available for assignment as a hot spare.  The question of format has come up, but because the disk was initialized by the controller and shows the new disk as a "Ready" status in the array, the new disk should not need formatting. Is this correct? We rebooted the server and the server came up fine and loaded Windows Server, and Dell Open Manager shows no problems with the storage and there is no warning of any disk being degraded. So what is our next step for setting up the new disk as a hot spare? Is is a format of  the new disk required?  How can we tell?.   The server is running Dell OpenManage Array Manager 3.7 Thank you for your help. Hammer
Trying to assign a dedicated hot spare array disk to RAID 5 virtual disk on 
legacy system - new disk not available for assignment