I know a local company that has been offering Seagate original disks to some of their Sun SPARC customers, all of whom report no functionality issues. In fact at one point they started offering Seagate 146GB disks as well, and advertised the disks as being tested on Sun 420/220R and 450/250 systems. Be aware that if your system has a current Sun support contract you will void the warranty by using non Sun drives. On the U320, it will work, I guess, but it will only operate at the bus speed that the on board controller will support - 40/80MB/s. Also the non Sun drives have a very different configuration and perform quite poorly with standard Solaris UFS configuration. One of the customers of this reseller recently critisized Solaris UFS as an underperforming filesystem when compared to Linux ext3 filesystem, but the test were carried out on non Sun disks designed for use in PC workstations without any additional UFS tuning.
<b>EDIT</b> I made an update here becuase the compay I mentioned no longer advertise these components for Sun systems.