Why is the SD card mounted as read-only on PC when shared from HTC Desire HD via USB? I just discovered that the SD card is now read-only when I use "disk drive" connection on the phone.

Why do I have to do in order to be able to write on the SD card from the PC?

I mention that the phone is rooted and is using the latest version of 2.3 available for HTC Desire HD (original rom).
It may sound really strange but the real issue was the corporate policy of mounting the the unencrypted drives as read-only. This is specific to BitLocker and there is a registry workaround available.