My iMac no longer sees my Seagate external drive since I upgraded to 10.6.8. What can In do?

First check the connections. Easy way to do this is by unplugging the cable and replugging it.

Next, open go to Applications / Utilities / Disk Utility. Whe DU is running, is the external drive listed on the left? If it is, select it - the item that is to the right in the list - then click First Aid, and Repair Disk.

After that quit DU.