Windows is writting a disk signature (serial number) to the Master boot record, if the two addresses 01B8 (4 bytes) and 01BC (2 bytes) are zeros.
You can export the first 512 bytes from the drive with dd. Then open diskmgmt.msc. After that, the disk should be initialized and the disk signature was written. Export again the first 512 bytes to another file and do a hexdiff of the two files. You'll see the different bytes in the above mentioned addresses.
Can I disable this behaviour in Windows, e.g. in the registry?
I'm on Windows 7.
diffs:
0000 01B0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 20 ........ .......
windows altered - signature filled in 01B8 (4 bytes)
0000 01B0: 00 00 00 00 00 00 00 00 72 44 E7 89 00 00 00 20 ........ rDþë...