I can’t access the USB hdd on my Mac

Hello,
I have iMac 2 GHz Intel Core 2 Duo, 2GB SDRAM, OS X 10.6.7 and for it I bought new USB hard drive.
My firmware is upgraded.
For back up I have used the FreeAgent. The main problem is that I see the backup, but I can't read it.
What should I do?
Dorothy

