You can never ‘simply add’ to a Library.

They chose the word ‘Library’ to denote a published work... think of your 124 Performances installed in a Library as a published Reference Book... first edition.
It’s technically in READ ONLY MEMORY. This literally means you can only “read” the data. You could no more change the Factory Presets (also in Read Only Memory). You are going to have to publish a “new edition”.

What you can do is the following:
With an initialized User Bank, place your four new Performances in the User Bank. 004/640
Use “Library Import” to add the Performances from the Library you wish to rewrite. (Importing allows you to “add to” the data currently in the User Bank”
Now your four new Performances and your (existing 124 Library) Performances have been placed in the User Bank... 132/640
Save the User Bank as a “Library File” creating a new .X7L FILE... if you wish to overwrite the Library File on the USB stick, you can target the same destination, you will be prompted “Overwrite?” But unless you are strapped for USB sticks, I recommend keeping old files (drag them off the stick and keep them in a folder on your computer).

You can now DELETE the old Library. [UTILITY] > “Contents” > “Library” > Select the old Library, DELETE it
Load your new .X7L FILE.

WHY: The ROM Bank is written (“burned”) into a very expensive kind of memory, Flash. Not called Flash because it’s quick to write into, it’s called Flash for the speed at which it is able to retrieve data (a big requirement for musicians)... your little USB stick can stream a stereo .wav file, but typically there’s a couple hundred milliseconds between you pushing PLAY and the file playing. If a stereo wave file can almost choke it you can imagine it’s not sufficient for musician’s real-time play requirements. (Imagine trying to play with 250-350ms between triggering a key and hearing a note).

The type of memory that is used to hold your Library data can stream 128 stereo channels, on demand... and it does so in very short order (faster than you can blink your eye). People who “don’t know” think ‘I just bought a 32GB drive for $10 bucks, how come Flash is so expensive?’... it’s that extremely fast retrieval time and that bandwidth.

Thanks BM! I feared as much - your analogy is very apt as always! I can't add a chapter - not even a single word - to a printed book. What you suggest is what I do now, and it's a bit time consuming (not a huge problem, just a niggle). But you've triggered another thought - why use Library files? In the back of my mind this 'over-writing' business persists, and it may be allied to User Banks ... instead of saving to USB as an X7L, save it as X7U ... the fact that loading it back to Montage wipes any existing User files would actually be an advantage to me - saves me having to do it first. I must think about this ... I believe I can over-write User Banks on the USB? As an option to another new Bank? I hear you about eliminating old Banks/Libraries, but I don't want an ever mounting pile of similar Banks/Libraries - I'd forget what's there anyway - but I would keep an updated back-up just in case I get another failed USB drive. A better angle for me perhaps, User Banks - more thought required! Libraries would be needed to avoid getting too unwieldy, for one thing, and to enable particular collections to stay together, unmolested by the Mad Mixer ... happy and healthy New Year to you!

Hello again BM - been experimenting today about this 'over-write' bee in my bonnet. Turns out I can do that with either 'Library' or 'User' files (as if you didn't know!), but because 'Library' files don't wipe out anything in the 'User' area when imported to User, they remain the safer option overall. In either case the USB file must be loaded back to Montage before it can be added to, so whether I save as a Library File or User File will depend on the particular circumstances. Thanks for making me think! Stay well ...

The fact that User files overwrite the user area doesn't make it less safe. The names of the files (User vs. Library) denote where the content is installed to when you LOAD the content. This is not an import operation, it's a load operation. "Load" is synonymous with "install" - although "load" is the official designation per the keyboard's nomenclature.

"Import", unfortunately, is somewhat of an overloaded term since the keyboard uses this term in two different contexts. This waters down any meaning "import" would otherwise have.

Montage first labeled an operation as "import" when you copy content from an installed (loaded) Library slot to the User bank. This use of the term is non-destructive and will leave all existing user bank content alone while adding designated (selected) Performances from the desired Library bank to the User bank.

Later, Montage added the ability to convert Motif XF Performance data over to Montage's native Performance format. Prior to this feature, only Motif XF "voice" data could be converted. It's this new feature - to select "performance" data vs. "voice" data - where the term "import" becomes overloaded and watered down. The interface calls the act of "loading" X3A data (Motif XF content), as one example, also an "import" operation. Before "import" only meant copying form a Library to the User area in a non-destructive fashion. Now, it also can mean to convert legacy data from either the Performance or Voice areas of legacy (Motif XF, as an example) content.

This splintering of the term "import" muddies the waters and forces us to be more specific if we are ever to now use "import" as a reference. You have to say "Library import" as one import or "converted performance import"/"converted voice import" as another type of import which designates the legacy data conversion type of "import".

Where "import" did, at one time, designate a specific direction of data flow (from Library bank to User bank) - this is no longer the case. Now it can mean from file to Library or file to User. Depends on context. Additional context is now required to avoid ambiguity.

Your mistake is equating “Import” with something non-destructive. (Fact not in evidence.) If you don’t make that misstep you are unlikely to have the issue you are having with the term.

If you simply take the term “import” to mean when you are selecting a specific data type to move into a memory location, you will not confuse the issue as you seem to have done.

While a “Library Import” (a Contents management utility) lets you select specific data from an existing ROM Library and move it into the User Bank, non-destructively, the “Import Option” (a Load Function) lets you select a specific type of data to LOAD from a legacy All Data File and move it into the Synthesizer destructively or non-destructively.

In the former, “Library Import”, it happens to be non-destructive to the current USER BANK... in the latter, you can direct the unit to go ‘into’ the All data file of a legacy product and select just the “Voices” or just the “Performances” from from a .X0A, .X3A or .X6A File and you can move them into either the User Bank (in which case they overwrite the current data as Loading to User always does) or you can direct them to a ROM Library (in which case they overwrite nothing).

Import does NOT equate to non-destructive, necessarily.

It simply denotes being able to *select* what gets moved. “Library Import” the data is already in MODX/MONTAGE format...when using the “Load” function to convert data from a legacy file, the “Import Option” let’s you select the data type from specific areas of the legacy file structure. If you direct it to User, being a Load operation it will overwrite the current User. If directed to a ROM Library, nothing is destroyed.
Legacy Files can be converted directly to a Library.

Loading to User is always destructive
Loading to a Library is never destructive.
“Library Import” is always non-destructive
The “Import Option” found on the Load screen is a Load Function, one that you can select whether it is destructive (direct it to User Bank) or non-destructive (direct it to a Library)

Hope that helps.