So I have five 3 TB drives in my unraid sever. One of them acts as the parity disk, while the others are storage. I was getting a SMART warning on one drive so I am trying to replace it with another 3TB drive, but it will not allow me to. Stating that the replacement drive must the same size or larger. Is this a bug or what?

 

Also I am replacing a WD Green drive w/ a WD Blue drive both 3TB

Read this:

 

          http://lime-technology.com/forum/index.php?topic=48303.msg463477#msg463477

and this:

 

          http://lime-technology.com/forum/index.php?topic=16541.msg151904#msg151904

 

HPA is usually the cause of this problem.  It happens behind your back and in most situations, it does not cause any problem but the parity drive for unRAID requires that it be larger than any other drive on the system and, if there is an HPA partition on the disk, that makes the remaining partition just slightly smaller. 

There are also reports of disks harvested from USB enclosures to be slightly smaller, it even happened to a user who bought them as internal drives from Amaxon or Newegg and got harvested disks, confirmed by checking the serial numbers on WD's RMA page.