Sorry it's taken me so long to get back on this.  I tried reproducing your problem and can't do it.  I have put textures in folders on two different drives (one external fire-wire drive and one internal SATA/2) and Carrara seems to be reading them just fine.  

Have you ever been able to read textures from the second drive in C6?  

My only other suggestion would be to reinstall Carrara.  Something might have gotten funky in it somewhere.

Good luck, and keep us posted; I'm curious about this one.

 - Dex