Code
if (sector_size != 512 &&
sector_size != 1024 &&
sector_size != 2048 &&
sector_size != 4096) {
sd_printk(KERN_NOTICE, sdkp, "Unsupported sector size %d.\n",
sector_size);
^ the error message in kernel code (drivers/scsi/sd.c) suggests the 256kb size is the issue so I'd suggest reformatting the drive to use at least 512kb or perhaps 1024kb and then restest to see what happens?