Make sure the Seagate external hard drive is connected to the computer, then do:
Right click on My Computer and choose Manage, then in the Computer Management window choose Disk Management on the left. You should now see your hard drives.
Select your Seagate External hard drive and right click on it and choose 'Change drive letter and paths'. In the next window that pops up, click the 'Change' button. In the next window (Change drive letter or path), use the drop down arrow next to 'Assign the following drive letter' and change it to another letter (like X or Y) and click OK. You will get a warning telling you that 'Some programs that rely on drive letters might not run correctly. Do you want to continue', hit Yes and that's it.
Don't do this on your system drives like your Local Disk (C

. It's OK to do it on pen drives and external drives where you are just storing data. If you run programs from these drives and the computer needs to know where the program is located, then doing this may make those programs no longer work as the location path would have now changed from E: to Y: for example; if that makes sense. I assign my pen drives and external drives their own letters like this as in XP it had a habit of not seeing it in My Computer sometimes, and I guess I just kept on doing it in Windows 7.