As Tuvok suggests, your existing E, F, G & H drives will almost certainly refer to card readers. Plugging in a USB pen drive might result in it being allocated to drive letter 'I' but, in my experience, it's far more likely that it will become 'E' (with the 'old' E becoming 'F', and so on).
Don't worry about it though. Simply plug in the pen drive and go to 'My Computer'. The new drive will either be 'E' or 'I'. Double-click on one of them to see what happens. If a window opens showing the contents of your pen drive (or nothing at all if it's empty), that's the drive you want. If you get an error message (telling you to insert removable media) you've got the wrong drive. Either way you can't damage any data.
Chris