Hi. I'm running Windows Vista 64-bit ultimate asnd I have the exact same problem. It will randomly write protect any one of the 5 installed hard drives as it sees fit to do so.
I spent ages scanning the web to find a solution, most people say delete partition then re-partition then re-format etc etc, this is all well and good but if this is happening on a reguilar basis to keep moving 240 or so gig from one hdd to another is both time consuming and realy annoying.
Anyway, I have found a solution to this problem. For me it works every time on and hdd without loss of data and spengina ages tranfering data. Although, obviously, make sure you back-up your data just in case.
go into command prompt
then type "diskpart"
type "list volume"
then "select volume xxx" (where xxx is the write protected problem drive)
then type "attributes volume clear readonly"
Then exit command.
Then try to write to the drive. All should be well.
This only didn't work for me once, when I did it again it worked fine, so I figured I must have made a typing error.
I thought I'd post this to help anyone out. It is a really frustrating glitch in windows that should be addressed urgently. Before I found this fix, I must had copied data, removed partitions, re-done partitions then re-formatted at least a dozen times. Hopefully this helps, please let me know.