Format ext3 disk space from Windows 7

It turns out that using the /q switch on Windows was the culprit: it enables 'quick format', i.e. the formatting process continues in background with every write made to the disk. Once it finishes, the drive is handled by Linux just fine.

Ah, the wonders of the SO/SF/SU trilogy: this question was kind of similar, but the answers didn't solve it. There was this comment however:

Reserved space for root user. Free space is free space for the unpriviledged user, not total free space. Linux doesn't display negative free space, unlike some other UNIX systems, when you start eating up root's reserved space. – pehrs Apr 16 at 8:42

According to this article, default space reserved for root is 5% of disk size.

By issuing tune2fs -m 0 /dev/md2 , the system now reports free space correctly (this system doesn't need root's reserved space).

The problem was not related to RAID at all.