You SHOULD NOT do software RAID with hard drives in separate external USB enclosures.
There will be absolutely no practical benefit to this setup, and it will just create risk of transcription errors between the mirrored drives due to any kind of problems with the USB connections, plus traffic overhead as the drives constantly update their mirroring. You will kill your USB controller, and/or the IO boards in the enclosures. It will be needlessly slow and not very fault-tolerant.
If this hardware setup is really your best option, what you should do is use 1 of the drives as the active primary for the server, and push backups to the other drive (with a properly configured backup application, not RAID mirroring). That way each drive is fully independent from the other, and the backup drive is not dependent on anything else. This will give you the best possible redundancy with this hardware.
You SHOULD NOT do software RAID with hard drives in separate external USB enclosures.
There will be absolutely no practical benefit to this setup, and it will just create risk of transcription errors between the mirrored drives due to any kind of problems with the USB connections, plus traffic overhead as the drives constantly update their mirroring. You will kill your USB controller, and/or the IO boards in the enclosures. It will be needlessly slow and not very fault-tolerant.
If this hardware setup is really your best option, what you should do is use 1 of the drives as the active primary for the server, and push backups to the other drive (with a properly configured backup application, not RAID mirroring). That way each drive is fully independent from the other, and the backup drive is not dependent on anything else. This will give you the best possible redundancy with this hardware.