My advice is:
1) Backup the contents of the drive as soon as possible.
2) From Disk Utilities, try verifying the drive, and repairing if needed.
3) If the problem continues, format the whole drive.
4) If it's still consistent, then be prepared for a drive failure soon. That sequence is so familiar to me.
I would add that at this point, it is worth spending an hour or so writing Zeroes to every block.
(that option is available on the Security Erase pane, as one click off the default). This forces the drive to substitute spare blocks it has in reserve for any that do not "come clean" after being re-written. If it errors out with "Initialization Failed", you can try it once more, but after that you probably do not want to trust that drive with your precious data.
If it never completes, it is dead.
I have 3 drives installed, 2 bootable. The error occurs when I boot to the Seagate 1.5TB (~850GB free) 7200 drive (my main drive now) or to the 250GB Maxtor drive. Does that still point to imminent drive failure?
Would getting a new 1.5GB drive, inserting it into an open bay, and cloning it be a good first step to save everything and be able to continue to work while I'm repairing the current drive?
The specs list a 750GB drive as the largest acceptable, but I've been running this 1.5TB drive for many years. Still, could that be part of the problem, a drive that is too large?
If you are running The Apple RAID card, the usable portion of the drives is capped at 2.2TB.
If not, there is no drive you can buy commercially at this writing that is too big. 4TB works. Everything smaller works.
One of the biggest overall performance enhancements you can make to your Mac Pro is to establish a Boot Drive, with only System, Library, Applications and the hidden Unix files including Paging/Swap. User files are moved off to another drive.
This stops the system's incessant paging and "snacking" for bits of this or that from slowing all your data transfers by moving the drive heads away from your data and back again.
I'm not running a RAID.
I think I understand your point, but first - Would getting a new 1.5GB drive, inserting it into an open bay, and cloning it be a good first step to save everything and be able to continue to work while I'm repairing the current drive?
Then after doing that I could reformat the other drives and create a boot drive on the 250 GB to improve performanace. Does that sound right?
Aside - Should I use this same strategy (dedicated boot drive with only System, Library, Applications and the hidden Unix files including Paging/Swap. User files are moved off to another drive) on my other MacPro (4,1 running OS10.6.8)?
Getting a new drive and making some copies seems like a good plan. I wanted to interject the Boot Drive concept while you were moving things around. You can implement it anytime if you choose to.
yes, Chris Pirrillo's recipe works. Some of the comments seemed negative, but the commenters' wounds were all self-inflicted -- there is nothing wrong with the Process. You just have to be sure that if you re-name the drive where you manually sent all the User Directories, you have to go back and manually change the references to the new Drive_Name.
Here is another illustrated recipe:
The way I like to set it up, you have your original Admin user, and use that only for Administration, not for daily use. That can be left behind on the Boot drive, since you hardly ever use it, and can be used in an emergency so that you can still log on if the User Data drive is sick. (you could just as easily create a new additional Admin User to be left there.)
Then you have a regular User for daily use, which is slightly less likely to get malware because it has no write access to privileged directories. That daily use account directory gets copied to a User drive and their pointers get adjusted (as described in the recipes) in Accounts. Once you are satisfied all is working, the original user directories can be deleted to make more space on the Boot Drive.