Multiple external Sata drives on an external sata port

I have a friend at work that has a Win 7 system with an external SATA port. He has used a SATA drive (drive 1) at this port with no issues. When that drive filled he bought a new one (drive 2) that he formatted and started to use. When he plugged filled drive 1 back in, it showed the folders for new drive 2 instead.

I told him to make sure that each drive gets its own letter (which he believes did not happen). He will change that tonight.

Other than that does anyone have any other insight?