Your i-device was not designed for unique storage of your media. It is not a backup device and media transfer was planned with you maintaining a master copy of your media on a computer which is itself independently backed up against loss. To use a device with a different setup you transfer the old library from a computer or a backup directly to the new setup, not the device to the library. Media syncing is one way, computer to device, updating the device content to the content on the computer, not updating or restoring content on a computer. The exception is iTunes Store purchases which can be transferred to a computer.
Redownload or transfer your iTunes Store purchases from an iPhone, iPad or iPod to a computer - https://support.apple.com/en-us/HT201267 - "This feature works only for content bought from the iTunes Store."
For transferring other items from an i-device to a computer you will have to use third party commercial software. See this document by turingtest2: Recovering your iTunes library from your iPod or iOS device - https://discussions.apple.com/docs/DOC-3991 Even this method may not fully recover what you had in the library originally. For example if in order to save space when syncing you had converted music files to a lower bitrate, or photos to a lower resolution, it is those lower quality files you will recover.
Better is to move your entire library:
Moving a library: Quick answer if you use iTunes' default preferences settings: Copy the entire iTunes folder (and in doing so all its sub-folders and files) intact to the other drive. Hold down the option (alt) key (shift on Windows) and open iTunes. At the select prompt choose the iTunes Library.itl file in the copied iTunes folder.
When this is to a new computer put the copied iTunes folder in the default location of Macintosh HD > Users > *User Name* > Music (or Home > Music) then you don't even need to start with the option key held down, iTunes will automatically look for it there. (Make sure there isn't anything already in the iTunes folder there that you want to keep since you will be replacing it with the one you are moving.) Also remember to authorize the computer in iTunes to your Apple ID if you use this to sync to Apple mobile devices.
More detailed instructions: How to move your iTunes library to a new computer - https://support.apple.com/HT204318 (but this one omits how you can start the library from other than the default location of Home > Music by using the option+start selection method.)
I presume you did not intend to post in the iTunes for Windows forum.
By the way, it is unlikely Apple wanting to do things this way. After all, it is in their interest to make things as simple as possible so people spend lots of money at Apple. However, it is the media companies who freak out at the idea of making piracy easy and they also get to say whether or not Apple may sell their media, so I suspect they have some say in software design.
