ZFS: can't read MOS of zpool...

Hello,

I can't say the precise point this started but I am convinced that was
right after running `zpool upgrade` after upgrading to 12-RELEASE:

---
ZFS: i/o error - all block copies unavailable
ZFS: can't read MOS of pool zdata
---

So, I got another disk to substitute that one, and now I have the same
message two times. :-(

https://ibb.co/G0sfv3p

I suppose that would be very unfortunate to have the same problem in two
completely unrelated disks at the same time; however, that error don't
occurs with the SSD.

Both disks pools were created raw, however I've tried to do using GPT, MBR,
and I got the same error messages.

---
ada0 at ahcich0 bus 0 scbus0 target 0 lun 0
ada0: <SanDisk SSD PLUS 120GB UE4500RL> ACS-2 ATA SATA 3.x device
ada0: Serial Number 1838DB806764
ada0: 600.000MB/s transfers (SATA 3.x, UDMA6, PIO 512bytes)
ada0: Command Queueing enabled
ada0: 114480MB (234455040 512 byte sectors)
ada1 at ahcich1 bus 0 scbus1 target 0 lun 0
ada1: <SAMSUNG HM320II 2AC101C4> ATA8-ACS SATA 2.x device
ada1: Serial Number S22KJ56S906708
ada1: 300.000MB/s transfers (SATA 2.x, UDMA6, PIO 8192bytes)
ada1: Command Queueing enabled
ada1: 305245MB (625142448 512 byte sectors)
ada2 at ahcich2 bus 0 scbus2 target 0 lun 0
ada2: <APPLE HDD HTS545050A7E362 GG2AB990> ATA8-ACS SATA 2.x device
ada2: Serial Number TNS519GY1G75LG
ada2: 300.000MB/s transfers (SATA 2.x, UDMA6, PIO 8192bytes)
ada2: Command Queueing enabled
ada2: 476940MB (976773168 512 byte sectors)
---

What more information would be useful? :-)

Thank you! :-)

-- 
Best Regards,
Alexandre C. Guimarães.
https://bitbucket.org/rigoletto-freebsd/