Sounds like the Seagate 1.5 TB drives are having some significant firmware issues, especially when used in certain RAID configurations. They tried blaming the Linux kernel for it even though the same issues appear to happen under Apple MacOSX and MS Windows. Here’s an example of the error that you see under Linux:
09:07:49 end_request: I/O error, dev sdc, sector 8
09:07:49 md: super_written gets error=-5, uptodate=0
09:07:49 raid5: Disk failure on sdc, disabling device.