I am looking to increase the size of a Windows NT RAID-5 set. How do I do it? Do I need to upgrade the size of each physical drive?

Thanks for your help in advance!

Is it software or hardware based RAID?

If it's hardware, you should be able to add a new drive, or upgrade each drive to a larger size (waiting for each one to rebuild first). Doublecheck with your vendor though to ensure one of these two ways works. It's how the ones I deal with on a daily basis.

If it's software, I don't have any advice for you on this. In my general experience, it tends to be less reliable if something goes wrong.

Before adding space, ensure you have two known good backups just in case.

If I understand the way that RAID works, each drive needs to be the same size. So, like Drakino said, you can simply add more same-size drives to the SCSI bus and tell the RAID controller to go use them. That's assuming hardware RAID on a SCSI system.

Hardware RAID is very nice. On the RAID system I just got up and running here, the controller (the one built into the Dell PowerEdge servers) did a fantastic job when I deliberately yanked a drive just to see what it would do. The management software politely informed me of the situation. Then, when I re-inserted the drive, it silently began rebuilding the replaced drive without stopping service. Took 8 hours to rebuild the drive, but hey, it's better than failing completely.

Okay, I think that Windows automatically handles this, but I could be wrong. Regardless, you can, in general, increase RAID systems' sizes by adding new disks (I sure that there are implementations that don't allow that, but I doubt they're common). However, increasing the size of the (pseudo-)physical disk won't automatically increase the size of the filesystem on it. That is, most filesystems create a static amount of storage when they're initially created, based on how much physical space they have available. When you increase the amount of physical space, the filesystem is not necessarily going to know to increase it's size, as well. But, then again, Windows might automatically detect that and Do The Right Thing^TM. Most OSes won't (for good reason).

If I understand the way that RAID works, each drive needs to be the same size

Some controllers support mixing drive sizes, and using them in a "JBOD" configuration where no space is lost. Others will allow mixed sizes, but just use the same space on each one corrosponding to the smallest drive. The controllers that support this usually support upgrading the array size by pulling each drive one at a time and replacing it with a larger drive. Once the process completes, new free space appears and can be used. Again, this all depends on the array controller. I know some that the above upgrading process is the only way space can be added to an existing drive, and adding a drive only allows you to create a new array.