Windows 7 permission on XP volume

Since upgrading my computer to Windows 7, I have noticed that Windows 7 doesn't behave the same way when slaving a hard drive from another computer system.

I often use a USB to SATA/PATA adapter to access a hard drive (usually the operating system drive) from my Windows 7 computer in order to either backup data or scan the drive for viruses.  When I would do this with a Windows XP host machine, I could access all the folders on the slave hard drive without any problems.  Now with my Win7 machine, when I access a hard drive, specifically the "documents and settings\%username%" folder, I get the message:  "You don't currently have permission to access this folder.  Click Continue to permanently get access to this folder."

I don't understand exactly why this message comes up.  If I don't have access, shouldn't it "challenge" me for a proper username and password?  If I click the "continue" button on the error message, it apparently just goes through all the folders and gives me permissions somehow.  If it's that simple to override the permissions, why even have this message appear?  And, why doesn't a WinXP host machine also bring up this error?

In my case, it is nice that windows security is so relaxed, so that I can scan or backup data, but if I ever lost my computer (laptop) what would prevent the thief from slaving my hard drive and accessing my data?  I realize there are additional security steps I could put in place to not allow this, but I just don't understand Windows security in this particular situation and why this error messages comes up in Win7.