Vista - Drives magically write protect themselves

I recently upgraded from Vista 32-bit to Vista 64-bit. I did this reformatting my C drive then performing a clean install of Vista 64-bit. I now have a problem with my volume(s) becoming write protected. I have to fight a constant battle against Vista to keep the volumes writeable.

I have two drives:
WD SE16 640GB: Primary or C drive
Samsung T166 500GB: Secondary or D drive (used mostly for storing downloads, music, video etc.)

Under Disk Management, both my drives alternate between being shown as "Online" and "Write protected": They can be shown as Online and then seconds later I hit F5 and both drives are suddenly write protected. Then I hit F5 again and they're "Online". Sometimes one of the drives is "Online" while the other is "Write protected". It's like a lottery, every time I refresh, the drives show a different status. It's like they're rapidly and constantly changing status.

My C drive always remains writeable despite this. I don't know why - maybe it's because the drive is in use by the paging file and windows system files so the change cannot be applied.

My D: drive on the other hand will write protect itself shortly after I have booted the machine. When I try to rename, copy or delete files I just get a Windows message saying the drive is write protected and I should remove the write protection or use another drive. When this happens I run Diskpart, and the command "Attributes volume clear readonly" on the D volume. This makes the drive writeable, but after a while (somtimes seconds, sometimes hours) the drive magically write protects itself again. As long as I keep modifying files on the drive it remains unprotected even if it sais "Write protected" in the disk management console. But it seems if I leave it idle for a while it will magically write protect itself and I have to do the diskpart thing again.

This is frustrating to say the least.

My user account has administrative privileges. I have tried disabling UAC. I have checked for virueses and background programs and tasks. I have tried about four different versions of the AMD/ATI AHCI drivers. I have checked the BIOS setup for any options that might be write protecting the drive. I have checked for Write Protect jumpers on both drives. This error is totally insane and I never experienced it with Vista 32-bit. It only started happening once I had switched to Vista 64-bit.

System:
Asus M3A32-MVP Delux AMD 790FX/SB600 mobo, Phenom X4 9650, 4x1GB Corsair XMS2, Radeon HD 4850, M-Audio Delta1010LT, WD SE16 640GB, Samsung T166 500GB, Lite-ON DVDRW (P-ATA)