Klennet Storage Software

USB and bad blocks

I've got a support call the other day, which ultimately led to me looking at the case via TeamViewer. The gist of the issue was that ZAR produced zero-length files. The drive was from some Iomega NAS, with no RAID to speak of. Some further quick checks ruled out permissions and UAC as possible factors. The initial scan was fine, and it did produce a correct directory tree, therefore I knew we were getting at least some data from the drive. However, by the end of the scan there were no files. So I started the scan again and this time I was watching the process.

The source drive contained two partitions, the small one with NAS firmware (that would be EXT filesystem most likely), and the large one storing data (that was XFS). Neither are recognized by Windows, obviously. The source drive was connected to a laptop doing the recovery with a USB adapter.

After a while, two boxes popped up, for D: and F: drives, along the lines of "Drive D:\ contains unrecognized filesystem", each one asking me to format its respective drive. This is a standard Windows behavior to ask to format any unrecognized drives you connect to it. The only thing wrong was that nobody did connect the drive - the drive was already connected.

Once I've seen this, it became immediately obvious that the USB adapter failed and was reset. Moreover, the target drive, where data was to be copied, was reset as well, because the USB hub, sensing that the adapter locked up, did reset all ports connected to it. This caused both source and target to be reset. Such things are most often caused by a bad block on a drive connected via USB. USB adapters do not handle bad blocks well.

There are two things to learn here

  1. Do not put source and target drives both on the same USB hub;
  2. If at all possible, use SATA, not USB. SATA does not choke when it encounters a bad block or some other malfunction. Better yet, short of a drive blowing up, failures of SATA-connected drives stay confined to that drive.