Back Up Ext. Hard Drive

I've been having problems with my external hard drive used for back up. Well to be more exact it is the software. As the hard drive is external it is only plugged in when back up is needed and my problem is that the software recognises the hard drive as "I" drive which isn't always allocated to my hard drive and is usually allocated to any USB device that i plug in first for example a mp3 player.

To over come this I had to allocate the end letters in the alphabet as Windows uses the next unused letter when mounting devices so now my drive is "Z" and the back up software is syncing "Z" drive so it doesn't matter what i plug in my computer first.