Reading file from external USB HDD is slow

Hi,

    I am reading a file (over 10 GB in size) in raw. I use FileStream in C# .NET to read the file. During course of reading the file, there are multiple seeks and reads done. The irony is that when I read the file from local HDD my application takes 15 mins to scan\read the file. However when I scan the same file from a USB External HDD, it takes 3 Hours. I am using USB 3.0. I tried disabling the antivirus to check if that could be a problem , but did'nt work. Can somebody please let me know what could be the possible reason for this huge difference in performance?