How do I unsync a device from all libraries?

Question

We have purchased a new computer and have relocated our entire music collection onto it, however we can no longer drag and drop music onto our iPhones at will. Two phones were used with the library and we've previously had no issued with dragging and dropping onto everything, however even though they both have the 'manually manage music and videos' boxes ticked we can't do this any more. When dragging and dropping, the cursor turns into the red circle and the file is not transferred.
I have tried disabling the automatic syncing features on the phones so that it stops interfering with manually trasnerring files, but it gives us the error message 'all existing apps /music and their data will be removed' which is far from ideal.
How do we enable the manual file sharing without having to delete everything from the phone? I'd ideally like the two phones to not be synced with any library so that we can freely connect them to any itunes on any computer and drag and drop the files we need, since there are multiple computers in this house and only being able to use them with one of them is a huge inconvenience. Mainly though I would like the ability to drag and drop files to be restored.

Answer 1

1. Be sure the phone are signed into the same iTunes account as the library on the computer.
2. Be sure the computer is authorized in iTunes.
3. Before syncing the devices, transfer the content in them to the new computer library (even though most, if not all is there already):
Disable automatic syncing on connection (as you already did).
Connect phone
Right-click the device tab on the left border of the iTunes window and select "Transfer Purchases."