Here is an excerpt from the FAQ page for dropouts in recording: It holds true for playback too:
– Avoid using long cables for the hard disk drive. Due to power loss on long cables, some of the hard disks go into a power saving mode and the read/write speed slow down, causing dropouts. Our USB storage connectors supply power to run external USB hard disks, but depending on the hard disk’s power consumption an additional power supply for the hard disk might be needed.
– Some hard disk drives have internal protection for damage when there is a lot of movement. When the music is too loud (especially with low frequencies) the vibrations to the hard drive may cause the drive to move its head to a parking position for a while. In this case the data transfers are stalled for a while causing dropouts. Please put a soft material under the hard drive to prevent the problems with vibrations.
If you have had no problems with a fast enough thumb drive, you could continue to use the same.