Toshiba portable hard disk drives

Intermittent issues are usually the sign of failing hardware. It could be a couple of things though, so you'll have to do some testing on your own.

Start by plugging the drive into a different USB port and see if that helps. Next you'd want to replace the USB cable between the drive and computer, and if that doesn't work, then you've narrowed it down to either the enclosure is failing or the drive itself is failing.

Regardless of any of that, now would be an EXCELLENT time to make sure any data on that drive you don't have backed up anywhere else, is backed up somewhere else ASAP.