Well I can try to give you some idea as to what I think might be going on but it's just a best guess really and it really only pertains to drives that were previously NTFS volumes since FAT32 volumes do not have a security tab.
When you add such a drive into a Win7 system the drive and its' contents comes with previously defined properties as they relate to SIDs and GUIDs (Security Identifiers and Globally Unique Identifiers) typically you will see these artifacts when you look at the Security Tab in the form of easily identifiable user or group names, which are unique to the system they are present on or in. Some may be a product of a default installation, while others may be explicitly assigned by you in the previous environment, while still others may be a product of something called inheritance which is a product of adding a file or folder to a folder which already has these IDs defined. For instance if you look at the properties of your C:\ drive at the security tab you will see some pre-set, pre-defined default security settings involving predifined security groups. Subsequent files and folders created in the root of C:\ will likely inherit these NTFS permissions while others will likely be added as a result of the creator/owner.
When you examine these same properties on a drive that you have added from an old system to a new one like you have done, these identifiers will appear in the form of a string rather than a readily identifiable user name or group name which indicates that the new system has no idea what they are or how to interpret what they mean in so far as local security settings are concerned and that is why you are likely to have to change ownership and explicity grant your (new) username the proper security permission to the file or folder in order to access it.
Hope this helps
Regards
Randy