Welcome to the PSR Tutorial Forum, akapy!
The PSR-A5000 has two separate internal Memory types:
1. The "internal Flash Memory" (~2GB) in which the "User drive" is located. The user-defined system settings are also stored here.
2. The "Expansion (Wave) Memory" (~1GB). The wave (sample) data of the voices from the individual Expansion Packs are stored here during the Pack Installation process. All other files from the packs (e.g. style files, registration bank files and the .vce files for calling up the Expansion Voices) are copied to the "Expansion" folder in the User drive during Pack Installation (i.e. NOT to the Expansion (Wave) Memory).
You can only see the current allocation of the Expansion (Wave) Memory in the YEM on the computer. I recommend exporting the Instrument Info file (.n27) from the A5000 to a USB stick again and importing it again into the YEM. (Delete the previous A5000 Install Target from the YEM beforehand.) The latest YEM version should always be used (currently V2.9.0).
If you click on your Install Target (PSR-A5000) in YEM and the packs that are installed (or should be installed) are selected, you will see the required "Voice Wave Size" in the top left (the value to the left of the slash). To the right is the maximum possible value (each in MB). By clicking on "Voice Wave Size" you can switch to "User Drive Size" to see how much space is required / occupied there.
Hope this helps!
Best regards,
Chris