gpart(8), as well as
http://www.wonkity.com/~wblock/docs/html/disksetup.html has some tips on alignment of GPT partitions.
In my previous post I aligned my disk on 63 x 8 = 504 sectors, but that is because I used MBR partitions and for some reasons buried deeply inside the FreeBSD kernel, FreeBSD refuses to accept MBR partitions not aligned on 63 sectors.
For GPT partitions this limitation does not apply.
It still could turn out that your disk has an hardware issue, but checking whether alignment of the partition helps would be helpful to figure that out.