Well, we now know the problem follows the stick. So:

1. either the stick is bad, or
2. the Samsung is writing it in a poor manner.

Repeat the experiment by writing to and reading from the stick on the Vista machine (I assume that machine has been performing satisfactorily, so we know it's OK). If the problem shows up there, it's definitely the stick.

If the stick seems to perform well with Vista, write a bunch of files to it from Vista and then see if you can read them on the Win7 machine. If all is well, then there's a problem with Win7 write drivers.

For your own satisfaction, try the write/read sequence with a different stick. And use different ports on the Win7 machine.

Something us definitely upgefuggled here, and with luck and a little perseverance we can find the problem.