Identical drives - Slightly less usable space - Mirror won't rebuild

I have two 250 gb WDC WD2500JS hard drives in a raid 1 mirror.  One failed.  I bought a replacement.  Mirror won’t rebuild as original has 232.87 free and new drive has 232.83 free.  Will reformatting or another step increase my free space so I can rebuild the mirror?

Backup your files to the new hard drive and then reformat the old one to the same size as the new one so you can rebuilt the RAID. 

Unfortunately the drive doesn’t just contain files but also has the operating system on it.   I suppose I could copy everything over and make it the boot drive - not sure I would successfully execute the change.  Difference in usable space - larger drive 232.87 gb - new drive 232.83 gb - that’s why I thought I should try and reformat the new 250 gb drive and perhaps it would yield slightly more usable space.