server 2008 - mirroring partitions

Thanks. We resolved the issue prior to reading your response - it was a bit of an emergency - but your response was along the right lines. While we did more than one thing which muddies the waters somewhat - we re-formatted the target disk - we did use the 'shrink' volume option in Windows 2008 to reduce the size of the 'source' drive by about 50 meg. When we did that the C and D were both available for mirroring onto the secondary.  I think there may be an issue here in that while the drives may be physically the same after Windows gets through with formattting/partitioning/convering to Dynamic, etc. on the source drive it may throw off OS perception of the equal drive size even though the drives are physically equal. The other alternatives are that the drives even though they are the same model actually are slightly different size so Windows actually reflects a slight physical discrepancy between the two or that the controllers they are on cause them to be seen to be different sizes in Windows even when though they are the same size. In either of these cases leaving a bit of free space may compensate for these factors.  For future  reference,  the drive was a 1 TB WD RE3 drive.  If you have any comments to my comments I wouldn't mind hearing.  I'll close the question if I don't hear back after a while.

Thanks for responding in either case.

I would like to add my guinea on your guesses concerning the actual size difference.
Normally both HDDs could be considered as 1TB drives but if one of them gets 1-3 bad sectors they are easily ignored by stopping file system accessing this zone of bad sectors. In other words the accessible space for formatting becomes smaller.
And bad sectors could be discovered even at vendor side before selling the drive. The space difference could be several kilobytes but for file system even a single kilobyte is important as you could see on your own example.
Hope my explaining is clear.

Excellent point about the bad sectors - I hadn't factored that in but it makes perfect sense. I don't know anything about the manufacturing process of hard drives but like anything else that's manufactured there must be a tolerance level for errors that go with any manufacturing process - e.g. manufacturing process  is tuned to have a range of  x bad sectors - let's say 2 -  as going to a a process that only allows 1 bad sector might drive up the price of producing drives beyond the value of the number of better drives produced.  In other words at some point the cost of additional 'quality' outweighs the benefit of the quality - for example a 'perfect' car that costs a million dollars.  
Just a note, if you're interested in hard drive stuff, there are some excellent videos about hard drive recovery on Youtube by Scott Moulton.  

Thanks. Yes I am interested in HDDs and all info referring to them is kind of my hobby.

Well, here's another tip you may or may not know about. It's a program called HDD Regenerator.  Excellent tool for fixing bad sectors - 'on the fly'  - on drives - I haven't seen anything close to it.  Do you have any favorite tools for hard drive recovery?  In particular, for hard drives that don't show up in the BIOS on boot - some kind of software that sees hard drives in the 'raw'?