I have done many an experiment with RBHP and pro upgrades, HD vs. USB and the like. There seems to be a priority to it all. For example, if I have the CON on the HDD and the PRO on the USB, works fine. Opposite? Not so much.

is you cache on the USB? When you reconnect your HDD, try moving the cache to the HDD before launching the game.

Due to the amount of DLC I want to upgrade, I tried to disconnect the HDD and read the upgrades package from the USB drive. Upgrades show up, but don’t work because the real DLC is on the HDD. so, I put the HDD in the Xbox and turn it on. Once the cache is built, upgrades should not be overrided, but instead they are!! Why??

There’s a very specific task sequence used for RBHP, assuming that’s what you’re using, and based on the description of the steps you’re listing I’m not 100% certain we’re getting everything done. Here’s a tl;dr version — make sure we’re hitting every stage.

Prerequisites

1. Same TU needs to be on your HDD and on your USB stick.

2. No song cache present on any connected hardware before we start — delete or back up any old ones before beginning.

3. Ideally, all DLC is on your HDD and upgrade files will go on a USB stick. If you’re not using the cache-rebuild method, that’s another can of worms we can get into.

1. Build RBHP packages.

2. Put dummy and upgrade packages on your USB stick in the proper folder.

3. Disconnect hard drive, connect USB drive.

4. Boot Xbox, boot RB3, let cache build. When prompted, I highly recommend saving the song cache to the USB stick. Confirm you can see upgraded songs showing harmony parts. They will have white album art/no preview, because it’s just the ‘dummy’ file.

5. Power down Xbox. Insert hard drive.

6. Boot Xbox, boot RB3, let cache build. Confirm you can still see upgraded songs showing harmony parts — again, they will have white album art/no preview, but should show the upgrades. If you do not, make sure the same TU is in both spots and there are no extra caches – in all but one case, this has been the cause for this behaviour (in the other, it was a weird case of certain pieces of DLC being on a USB stick and not loading properly).

7. Quick to Dashboard, go to System Settings > Storage and delete the dummy file off the USB stick. This is the smaller of the two RBHP files. Do not touch the song cache or the larger RBHP upgrades package.

8. Boot RB3, let cache load. Verify songs have upgrades. Play.

If you are just using pro guitar/bass or pro drum upgrades, you should be able to drop the files in your usual customs (RB3 saved game) folder and play without the cache stuff, but because we have to lie to the game about harmony parts (and keys parts), the above has to be done.

If the game is not showing a pro part and crashing on load, make sure it’s not a song where there is an RBHP upgrade and another pro guitar upgrade (either official DLC or custom) for the same song in your setup at the same time. You need to merge the two together to get them to play nice, as they cannot co-exist.