Yes, you do have a problem. Are these mirrors Data or System? When you boot the NT Server, does the system come up stable and well? ALso, what do you mean by initialize? Are they reformatting every boot?
Are you running the latest SPack? I think it is 6a. If not, do not just run out and upgrade.... it might break something.
You may forage into the Event Viewer and into the Disk Administrator and see what is going on. Do the logs suggest any problems? Do you need to do a checkdisk and clean them up? Overly fragmented?
You might have to break the mirror set and check them independantly too.
But before I went too far, I would get a SOLID BACKUP of that mirror set. Have a spare drive handy? break the mirror, put the new drive in there. Make a new mirror. Once that is stable, break that one, and work with the first two disks. Put that third drive on the shelf in the event of Armegeddon.
I am running sp6, and this is a new setup, so right now no data is on the disks except the OS. When the disks are first mirrored they got "initialized" according to disk admin. This happened once after it was first done, and it was fine for a couple of restarts, but now....and the process takes about six hours. I don't know if it matters but the disks are set up master and slave on IDE1.
I know that mirroring works with IDE and NT4. I won lunch over that discussion a couple years back.
What I think you need to do is break the mirror, and then format one drive to a size a bit smaller than the other volume. Then establish the mirror.
Disks come in a variety of sizes. You may have purchased an 80 GB drive, but it might format to 79.43 GB. If the other drive formats down to 79.38 GB, then you have a problem. I would round it down to the nearest integral GB for safety.... 79 GB. Yes, that is some waste, but drives are not identical to each other, and if you have to replace one, the third drive might be 79.25, and well, you are stuck.