Few years ago I had a collection of maybe fifteen old disks, which I wanted to get rid of, by means of recycling. First I wanted to check the content and then format all so I put them in an external enclosure. It turned out that some disks were unusable. A closer inspection showed that these were all a certain brand and type (Forgot whether it was Seagate or Maxtor or WD). These disks would probably still do fine in a desktop or server computer (Which I no longer had at home) but not with the external enclosure. Perhaps your enclosure is the bottleneck here as well.