It depends on whether you're talking about server-class gear or desktop-class gear.
If it's a desktop machine built with your own money and off-the-shelf drives, and you're not worried about compatibility, then yes, your strategy is sound. Every X years, go out and buy all-new drives to replace your current drives. They're going to be faster, quieter, and larger. You could replace the drives individually, letting the array rebuild itself, and then when the rebuilds are complete, reconfigure your array to be larger. (Not all raid adapters support operations like this - online rebuilds and size changes.)
If it's a server-class machine like an HP Proliant or IBM System X, it gets more complicated. You may need to use hard drives on the compatibility list for your raid adapter. In that case, the drives are going to be expensive because they're probably no longer produced, or they're just plain expensive to begin with on server-class stuff anyway. Even worse, you might be buying refurb gear from your reseller and not knowing it - this isn't uncommon with server resellers.
Plus, you may be discarding drives with perfectly good lifespans and replacing them with drives that are destined for trouble. Rather than proactively replacing those, it makes more sense to build the server with a hot spare to begin with, and make sure your raid array supports automatic rebuilds using a hot spare. Then the rebuild will happen before you even get out of bed to make it into the datacenter, and you can replace the dead drive at your leisure without spending money or time.
I believe you are out of luck. This is one of the dangers of RAID5. Since the array was in use, all the other disks are now out of sync with the original port 10 disk.
updated: Regarding the update read-only mounting...Whether or not this works is really going to be an implementation detail of the 3ware. Even if you mounted read-only, the raid controller could have updated some metadata on the disks and decided this configuration is not recoverable. That's what I would expect.
Best Answer
I have done this in the past and just replaced it with a larger drive. Is there any reason you can't use a 250gb version of another drive in the same manufacturer or model?
That will work as I have done it many times.
Example I had some 1.5TB drives fail, I just replaced with similar model drives but 2TB versions. Rebuild worked just fine. Correct me though if I am wrong.
Yes this was all done with 3ware raid controllers under raid 1 and raid 6 using 9650se 3ware part number as the hardware raid controller.