2 External drives with the same error

Why would I get the same FSCK error on 2 different external drives? About a year ago I archived some important data (photos, etc) onto a 3TB external drive and then copied it (terminal copy command, not an image) onto a 2nd drive. The 2nd drive was stored in a different location to the first. Both drives are formatted HFS+ I recently plugged in the drive to retrieve some files and whilst the drive appears to work fine (photos all load, videos load), OSX says the drive is dirty and I get the below error when running FSCK. The drive definetely was fine last time I used it, as I ran FSCK on it before storing it. ** Checking catalog file. ** Rebuilding catalog B-tree. hfs_UNswap_BTNode: invalid node height (1) hfs_swap_BTNode: offset #0 invalid (0xFC11) (blockSize 0x2000 numRecords 24) Invalid node structure (4, 12830) ** The volume 3TB could not be repaired. Now it gets weird as I just checked the 2nd drive and it has exactly the same error in FSCK despite being a different model (seagate vs WD) and it wasn't created by image, it was populated by an OS level file copy. I don't need help getting the data off it, but I love to know **why** or **what** is going on here? My extensive google search turns up either "physical issue", or "just run disk warrior". Whilst possible, I doubt its a drive hardware issue. Could it just be a bad file on the disk? Or a hardware issue with the USB interface (yes I have tried different cables)? Thanks