Reading data CD: please help!

Hello everybody,

I have a CD-R disk that contains file A with original CRC Z1.

Step 1. I put the disk in my drive, and read file A. The file is read without any errors, but it has CRC Z2.
Step 2. I put another disk into the drive, then replace it with the first disk and read the file A again. The file now has CRC Z3.
Step 3. I put another disk into the drive, then replace it with the first disk and read the file A again. Finally the file has CRC Z1.

The problem: CRC Z2 and Z3 are not equal to the original CRC Z1, though the file was read all three times without any errors.

The question: how is that possible? And more important, how to solve this problem?

The drive is NEC ND-1300A.

The problem is not related to my hard drive, because I tried to calculate the CRC without copying the file to the hard drive and the problem did not disappear.

Please help!