Update:
After some further copying moving, etc. of files on the 3TB NTFS GPT partition, strange errors occured on a chkdsk run and defects in the filesystem exist now.
So, if a RDM passhtrough'ed disk shows the wrong size, care must be taken, something goes wrong, even if a already existing partition and filesystem looks good.
Any idea, where's the problem here with the SATA drive?