I had this problem since version 8 (tried today with 8.01 RC1) but thought always this was my mainboard (a VIA VB8001). Today, new hardware (ASRock E350M1) I had the same issue: I am unable to create a working zfs volume. First, I have to execute 'sysctl kern.geom.debugflags=0x10' on a root shell, otherwise I get 'Unable to GTP format disk ada0'. After executing that, I can create a ZFS (mirror with 2 disks, WD20EARS), the "use 4k" ckeckbox is checked, of course. All seems fine, I can access the volume, write an read data to/from it. But after a reboot, I am not able to access the volume. dmesg says: Code (text): ada0 at ata2 bus 0 scbus0 target 0 lun 0 ada0: <WDC WD20EARS-00MVWB0 51.0AB51> ATA-8 SATA 2.x device ada0: 300.000MB/s transfers (SATA 2.x, UDMA5, PIO 8192bytes) ada0: 1907729MB (3907029168 512 byte sectors: 16H 63S/T 16383C) da0 at umass-sim0 bus 0 scbus4 target 0 lun 0 da0: <Mass Storage Device \\001\\000\\000?> Removable Direct Access SCSI-0 device da0: 40.000MB/s transfers da0: 3826MB (7837695 512 byte sectors: 255H 63S/T 487C) ada1 at ata3 bus 0 scbus1 target 0 lun 0 ada1: <WDC WD20EARS-00MVWB0 51.0AB51> ATA-8 SATA 2.x device ada1: 300.000MB/s transfers (SATA 2.x, UDMA5, PIO 8192bytes) ada1: 1907729MB (3907029168 512 byte sectors: 16H 63S/T 16383C) GEOM: da0s1: geometry does not match label (16h,63s != 255h,63s). GEOM: ada0: the secondary GPT table is corrupt or invalid. GEOM: ada0: using the primary only -- recovery suggested. GEOM: ada1: the secondary GPT table is corrupt or invalid. GEOM: ada1: using the primary only -- recovery suggested. GEOM_MIRROR: Device mirror/mirror0 launched (2/2). GEOM: mirror/mirror0: corrupt or invalid GPT detected. GEOM: mirror/mirror0: GPT rejected -- may not be recoverable. I had it all up an running on FreeNAS 0.7 without problems, but version 8 with zfs is unusable because of this. I suspect the disks (advanced format) may be the problem... If I put one of these disks into my linux box, I can import the filesystem without any problem (aside the missing second disk) using fuse-zfs and access the data. If I use UFS, I first (without the geom.debugflags) get the error: Code (text): Error: [MiddlewareError: Volume creation failed: "geom: Can't store metadata on dev/ada0: Operation not permitted., "] After doing the debugflags trick, it works and it does so even after a reboot.