Because MSDOS partitions can start on sector 32 and GPT partitions start at sector 64, there is a condition where a pool that uses the entire MSDOS device cannot be mirrored to a GPT device of the same size. This problem occurs because the free space on the GPT is 32 sectors smaller. The NLVM command reports an error, and fails to mirror from a DOS partitioned device to a GPT partitioned device of the same size. In NSSMU, the GPT partitioned device is not presented as an option.
If the device has other partitions on it, the problem should not occur because the first partition is already adjusted for the 32 sector difference. It should also work if the GPT device is larger than the DOS device.
To avoid this problem, when mirroring a pool that consumes an entire MSDOS partitioned disk, the device used to mirror it must also be MSDOS if it is the same size. If the device is at least 4 MB larger, it can be either MSDOS or GPT.