The way Euphony does updates to its system is based on having two system partitions:

• active one - which you boot from
• inactive one - which you use during update 

Update essentially downloads the whole system partition filesystem image and applies it to inactive partition. Once that is done the system is simply told to boot from the updated partition. The previously active partition becomes inactive (and retains previous release to which you can always return to).

Important: when you burn the Euphony image to USB and boot from it, you only have one system partition, the one you booted from. Inactive partition still does not exist.

To be able to receive updates, you first have to register (Trial or full paid registration).

If you register for Trial and you want to receive updates during the trial period you have to create this second, inactive partition. This is done by the 'Complete installation' function. Complete installation will create this inactive partition from a filesystem image that is already present in the filesystem of the image you burned to your USB.

Once you fully register, you will now have 2 options: 

• Install - installs Euphony on another drive creating all the needed partitions and restoring the filesystem from the filesystem image to both system partitions. 
• Complete installation - creates only a second partition on the current drive like before thus enabling updates 

Both of these options will work and behave in the same way if everything is in order. But in case of problems, Install option is preferable ***.
*** The remaining part of the article is applicable only for Euphony versions before v3.20200114. As of v3.20200114 this no longer applies - 'Complete installation' function also replaces the default partition UUID of the primary system partition making the problem described below go away.

Potential issues have to do with partition UUIDs. In Euphony we use UUID as main identifier for mounting partitions. Default system partition in Euphony image that you burn to USB always has the same UUID: d85fe591-2026-4e75-a314-44d5c435c372

If you run your Euphony from a USB or other drive that you created by burning Euphony image to it, you will always have that same UUID for your first system partition.

If something happens to your installation (corrupt filesystem or some other unforeseen error) and your system becomes inaccessible, you usually have the option to boot from USB with fresh Euphony system image and run Install/Repair with your primary drive as destination. Install/Repair will preserve your music data and you can again have a working Euphony installation with relative minimum fuss.

However, if you use Complete installation, your corrupt drive will have the same default UUID for its system partition as the new USB drive you created for repair purposes. This will cause conflict, your system will behave unpredictably and you will not be able to do the Repair procedure. You will have to install everything from scratch and lose the music that you copied to Euphony Drive and all metadata. If you burn the Euphony image on an internal drive you may have to find a way to erase it completely (partition UUIDs must be deleted) to avoid that conflict so you can again boot Euphony from USB.

If you had used Install, newly created partitions would have got new UUIDs with no chance of UUID conflict and you would always be able restore Euphony on your internal disk by using USB drive with Euphony on it.

That is why Install is the best option.