Unfortunately no warning whatsoever.
If you have another 1TB drive, do a sector level clone of the disk using a disk ghosting tool. You should then be able to restore your disk if the worst happens.
Please note that this must be done at the raw disk level, and not using any tool that inspects the FAT file system and attempts to be clever by only backing up used sectors.
A quicker option that offers some protection against a reformat would be use the backup utility
I have an equivalent reverse tool that restores such a disk, which I could send you. While this is not a complete backup (it does not backup any of the actual video from the recordings) it would allow a restore so long as no new recordings had been made on the new disk in the meantime (watch out for Anytime and Instant Rewind making "recordings" behind your back).
The principle is that it restores all the other disk structures (file system, files etc) in the same place they were previously and relies on the actual video data not having been overwritten in the meantime. But if new recordings had been made, and those occupied some area of the disk previously used by old recordings, then they would be overwritten and those effected recordings would be unplayable - either in whole or part.
Note that when a Sky disk "reformats" a disk it only ever does a quick format (by re-writing empty disk structures) and never by physically zeroing every sector on the disk (which would take far too long).