No idea what's happening during your rebuild, but if the first step of the rebuild is to do a verification of the drive and the drive "fails" for whatever reason -- too small, too big (in some cases), bad sectors, bad interface, etc. then that could be why you aren't seeing activity -- you didn't get far enough to START the rebuild process before you fell over.
Boring story -- we have an ancient RS/6000 here that's running with a mirrored OS drive. One of the 2 drives in the mirror set died. No problem, we're under IBM hardware maintenance, and they quickly showed up with a new drive, free with physical installation included. Unfortunately, the new drive is twice the size of the old drive -- the old drives are no longer available through IBM, no way, no how. Unfortunately, it's impossible to format said drive with the same cluster size as the existing drive....the machine insists upon allocating the whole darned drive, and when it does so, it requires a cluster size that's twice as large as the existing drive's geometry. So, it will not, under any circumstances, allow me to mirror to that drive, period. Can't even tell it to use the old cluster size and cut off the top half of the drive (which should work in theory) because the routines won't allow me to NOT use the entire drive. Grrr.....Now, IBM would be happy to sell me a matching "too big" drive if I didn't mind having to save/restore my boot partition to get it to work. Gee, thanks. Ultimately, we went through a 3rd-party channel and purchased a matching "old small" drive at our own expense. Actually, we purchased 2, just in case...
Anyhow, the moral to the story is, at least for some machines in some situations, sometimes you HAVE to make sure that the drive geometry is compatible when replacing a drive in a RAID set. Even when it doesn't strictly make sense that it should have to.
BittMann