Your enclosure may have some circuitry specifically designed to look for certain HDDs. If it fails to find those models, it may assume that there is an error. Seems unlikely though.
When you installed the external device initially, did you install any software with it (from Toshiba)? Also, have you looked at your disk manager (control panel/administrative tools/computer management) to see if the new HDDs are seen but not partitioned/available?