USB attached drive wont boot, but does when internal

[TheDon]

I have a working 10.5.2 120 gig drive and a backup 60 gig drive. The 120 gig is my internal laptop drive. the 60 is the same type of drive, but its loaded in an enclosure. The first time I did a drive copy using disk utility/restore everything worked fine. I could boot from the extra 60 gig drive while attached in an enclosure with USB (with or without my 120 gig drive in the notebook). But now weeks later, it boots up part of the way and then the drive light stops and nothing happens. I can boot with safe mode. I get everything but sound and itunes coverflow at that point. I recopied everything with the restore tonight and same thing happens. Then I took that 60 gig drive, pulled it out of the enclosure and put it in the laptop itself and it booted fine. Why is the USB boot not working for me? It worked before.

thanks for reading and any advice.