Very likely this is a cabling issue if you experience this problem since first usage of the external enclosure. Whenever high speed data transfer is done with inappropriate cable, interference between cable can disturb the transmission.
There are three way to solve this:
- Get a properly grounded and shielded USB cable, the interference between wires will be minimized.
- Reduce the length of the USB cable, this will reduce loading effect and keep harmful interference way below significant level, thus reduce the rate of fail transmissions.
- Get a ferrite cable clamp kit and attach it to the USB cable, this will adsorb the EMI high frequency energy in the cable.
In the example here is a faster approach for me, reduce cable length! This is original cable come with VZTEC cheap SATA enclosure I get from SRI Computer at RM20, how come they sell a product that have such an obvious defective in design to average consumers? A picture worth a thousand words, here is two thousand words...
Isn't that easy? Just cut it and join it together! Enjoy a healthy transferring mode that never annoyingly disconnect itself anymore, good luck. 
