Do not mount file systems on a metadevice's underlying slice. If a slice is used for a metadevice of any kind, you must not mount that slice as a file system. If possible, unmount any physical device you intend to use as a metadevice before you activate it. For example, if you create a trans metadevice for a UFS, in the /etc/vfstab file, you would specify the trans metadevice name as the device to mount and fsck.