Vista - usb mass storage device problem

i had a friend "mount" (correct term?) the harddrive from my old
laptop(compaq presario, circa 2003) and it is now repurposed as a USB mass
storage device. it contains all of my important files from my old laptop.i
have not plugged it in about 9 months. i just plugged it into my new dell
inspiron laptop. device manager says "device functioning properly." i also
have plug in/out sounds and a "remove hardware safely" icon. but i can't
access my files or save anything to the device. disk manager says calls it
"disk 1, unknown" and say it is "unreadable." why can't i find my device on
my desk top to open it/save on it? why is it unreadable? it worked 9 months
ago. i have moved around alot, but i do not believe it has been damaged.
please help!