That really depends on how you put them on that drive. Here's one option:

1. I put them on the drive by running 7zip, selecting the files and zipping to a file on that external drive.
2. I get them back by running 7zip, opening that file on the external drive and unzipping to a folder on my internal drive.

Of course, there is no real need to compress 30 Gb of data if your external disk is 1 TB. But that's your choice.