I purchased two external drives of 5Tb from Western Digital. I formatted the drives using disk utility program (OS with journalling). I connected the first and started to make backups using time machine. I thyen connected an unrelated disk to transfer my movie library from my macbook pro to, to diminish the data on mu macbook pro. This failed, somehow the unrelated drive seemed to disconnect by itself, thereby unabling the transfer of data. I thought maybe something was wrong with the connection of this drive to my computer. Therefore I decided to use the second 5Tb drive for this large movie library file. When I connected the second 5tb drive, it immediately indicated that it was dedicated to time machine, but I did not enter this drive in the TM preferences. I could not undo this, and the drive was not available for anything else. When I connected drive 1 again, it was not recognised by my mac any longer. And thereafter neither was drive 2. When I checked about this computer I could see that the drive was listed under USB. And when I opened disk utility it was seen however no modifications nor disk first aid could be done. Basically both disks were suddenly totally useless. I posted this on a facebook page and someone suggested: could mac os incidentally have put this disks in raid. What happened, does anyone have a clue?