When trying to download music, the SD card does not mount. I have to shut the phone off and reboot, then it will see the SD. This happens after every download of music.
VCAST Media manager is loaded and all other needed apps. SD has been checked for proper position on the phone.
Not sure exactly when it won't mount (on the computer, or in the phone) or by what you mean "download music" (transfer music to the phone, or actually download a music file OTA to the phone from Amazon etc.)
I'm taking a guess here:
You attach your phone to your comptuer with the data cable, mount the SD card from the pull-down menu, copy music to your phone. (As opposed to actually downloading music to the phone via amazon or similar.)
After you transfer music, pictures etc. to the phone, you have dismount the phone from the PC before your Droid can see the SD card. This is done by "pulling down" the menu bar again, clicking on the notificaiton icon and dismounting the SD card. Android can not (by design) read the SD card while it is mounted to the PC. Until you unmount it, or reboot it which will do the same thing, your phone will not be able to play the music, view the pictures etc.
If you are already doing this, then it is likely the SD card is becoming corrupted because windows is telling you the file is transferred already, when in reality the system is still transferring data in the background from it's file system cache. On your pc (not the phone) look down at the notificaon bar near the time clock. You will see is little symbol with a green arrow that indicated attached USB devices. If you left-click it, you will see all USB devices attached, click on your Droid to dismount the USB card from the computer first. It will flush the buffers in the PC, forcing it to complete the transfer then tell you that it is safe to remove your USB device. At this point, pull down the notification window on the phone and dismount the USB card. This will assure that there is no corruption on the SD card due to incomplete transfer and "closing" of the files.
Every time the phone is rebooted, it does a fsck (File System ChecK) on the SD card which would *usually* repair any corruption on the SD card, so this might be your problem.