hey, welcome to Daniweb!
You're right, theoretically this should be a very simple fix. But remember, in the IT field things go as planned about 5% of the time.
To get your documents in one place:
Open the external hard drive and look at the files to ensure all is there and well. Minimize that and goto Start->My Documents and look at the files to ensure all is there and well also. Once you're confirmed the data you're working with you could simply highlight everything on the external drive and drag it over to the My Documents folder.
things to lookout for:
1. When prompted you'll click "Yes to All" to overwrite all the duplicates BUT be careful because if you do, any versions of the files on the external drive that are OLDER than the file to be overwritten on the local drive will OVERWRITE that file.
2. Make sure you have enough space on the local computer to save all the data from the external drive.
Be very careful with #1; make sure you read it carefully

If I didn't explain it well may you could ask and someone else could do a better job.
also dealing with your mapped drives. Look at the letter of a mapped drive as nothing more than a label; a place holder. Since everyone has C as their primary drive it wouldn't make sense to make C go to someone elses computer, right? That's why when mapping
other people's drives to your computer, you usually start at Z. Does that make sense?
sorry if this isn't very detailed, i'm on my way out. let me know if this didn't help at all lol