Error when importing tracks to RB

For some reason, when I import some tracks into RB it states they cannot be found, and I get the yellow triangle symbol. Strangely, this does not happen with all tracks, and I get the same outcome when dragging from iTunes or using the Bridge feature. Why?!

N.B. I have never had this problem before, and have used RB for over a year. Also, all my tunes are stored on an external HDD.