HD failing, would like bigger, Can I?

You can use either another 1.5TB drive or a 2TB drive. The NV+ (and Duo, both Sparc processor based units) will only support drives up to 2TB. Only 1.5TB of the 2TB drive will be used. On the NV+ the capacity is limited by the size of the smallest drive. Only if all are replaced by 2TB drives will the additional space be recognized.

I would back up all my data that was critical or important and then I would shut down the unit, remove each drive in sequence connect it to an SATA port on a desktop motherboard and run SeaTools long generic test. After that if all the other drives passed with flying colors, I would restart the unit and once it rebooted and was up, I would hot remove the problem drive, wait for the unit to recognize that a drive had been removed, and then hot add the new drive (after previously running SeaTools on it as well.

Please note that the long generic test of SeaTools takes about 2 hour to run completely. The reason for the caution, is that you may have another drive that is weak, and when you add a new drive, the stress of the long resysnc (8 or more hours) can cause a second drive to fail, which puts you in the unpleasant situation of a dual HD failure which can cause loss of data. If you have a full and complete backup of your critical/important data, and cannot or do not want to run all the SeaTools tests, if you have problems, at least you have the backup to fall back on. (Restoring up to 4TB of data is not quick and easy either.