I have had similar experience and found that due to having a large media library in the hard disk in question, my issue was not what i previously thought.
The Windows media player was set to re-organize and or rename files according to my Rip disk settings; This acts like a write protect function if any write-protected media is present. and crazy as it sounds that will cause you to get the "Remove Write Protection" error telling you that disk or volume is write protected.
what i found to fix this was to remove the settings in media player allowing it to re-organize and rename files, after which i went to the tools/advanced/restore media library and allowed it to dump my current library and reset it.
after doing that it is often accessible, if not then try a reboot to clear the settings, or use the DISKPART functions to clear the write protection.
I giggled to myself when i started doing google searches and found soooo many old posts that were also in vista forums about the same kind of thing, that were probably the same fix.
I would be interested to know how many find this to fix the issue for them btw