ZFS pool degraded on reboot
You should not use /dev/sdX names for your pool configuration.
Any change in SCSI enumeration, say inserting a CDROM or a USB drive, could cause the device names to change, resulting in the errors you're experiencing.
You have an option of using the /dev/disk/by-id names.
Do this with a zpool export tank and zpool import -d /dev/disk/by-id tank