There are different types of memory metrics in MODX. I'm assuming you mean sample memory free - although there are also other memory limits such as number of Performances (in user area), number of Libraries, etc. Most of these can be inspected from the same general area (Contents, Data Utility) by looking at different folders.

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For sample memory ...

Source: previous message from BM for Montage - menus are the same for this. Edited to match the MODX (maximum waveform size only).

f you already have Libraries installed then you must have an available Library slot and enough memory remaining to accommodate what you are attempting to install. If not, you will receive a Sample Memory Full message.
You can view how much of your Sample Memory is used by navigating to your WAVEFORM folder:
[UTILITY] > "Contents" > "Data Utiliy" > touch the "Waveform" folder; You Sample Memory used out of 1.0GB (for MODX, or 1.8GB for Montage) will be shown in the upper right corner.
Verify you have enough memory left before loading the file.

I want to import Chick Mark V (the single performance) into my User bank so I can delete the Chick library.
However, just trying to import that one performance gives me "Sample memory full.,"
Well, my sample memory is full because the Chick library is so big.

I deleted several custom samples that I probably won't use again but it's not enough. I'm down to 809 MB in my waveform memory, but it's not enough to import the performance.
The handful of custom samples remaining are things that I use regularly.

I would LOVE LOVE LOVE it if we were able to grab a single performance and its associated samples from a library on a USB drive without having to install the entire library.

Whether you import one Performance or all sixteen Performances from the Chick Corea Mark V, the amount of Sample Waveform data is exactly the same.
Just FYI: Stereo audio samples use about 10MB per minute... meaning the Sample Waveform (Oscillator) data is very large.

Many, many. many Performances can be built from the same set of sampled data. The sampled waveforms of your MODX are playing the role of the waveforms found on an analog synth. The analog synth had a Sawtooth, a Pulse(Square) wave, and maybe a Triangle wave... from the one Sawtooth waveform - thousands of different sounds are made with this one waveform. Same with sampled waveforms loaded to your MODX -- you can build many, many Performances from the same data. Rhodes with Chorus, Rhodes with Flanger, Rhodes with Auto Pan, Rhodes with Dual Chorus, etc, etc., etc.

The Chick Corea Mark V is approx 400MB of data... the parameters that create each the 16 Performances use no Waveform memory (and are tinny tiny in size comparison)... we're talking about the parameters that shape the audio samples into playable sounds only amount to a couple of kilobytes of data. (And are stored in internal memory)
Use dollars (or currency) to appreciate this difference in data size:
$400,000,000.00 as in 400 million dollars -- the approx. data size for the AWM2 Oscillator.
versus about $2,000.00 as in 2 thousand dollars -- the approx. data size for all other AMW2 parameters settings in a Performance
HUGE difference in size... So having dozens and dozens of sounds made from the sampled data does not use much memory and certainly takes only seconds to move around.

So whether you import one Performance or all 16 the Waveform size is the same... ALL of the Chick Corea Mark V Performances use the same exact Waveform set.

Ok thanks, Bad Mister. I didn't realize that all the sample data was required for that Performance.

Is it normal for my User library to be 400MB when I only have about 10 MB of custom samples?

Some food for thought...

1Gb of User Memory is plenty, but you have to remain conscious of the fact that it is a limit. Good House Keeping should be employed regularly.

Think of it as any common household Wardrobe or Closet. It seemed like ample space when you bought it, but now you can't fit anything in. This is likely because it is full of junk you used once, you never use anymore, nor are you likely to use ever again. Not only that, you can't even remember what's in there.

The recommended method to avoid this is regular Backups and Purges. A USB Memory Stick can store 64Gb or more... never mind Wardrobes, that's like having a "Spare" Warehouse.

My method, prior to a Purge, is to:-

1) Save "Current" User Presets (Single Performance Files). They are "current" because you still use them a lot, or are still an "under construction" project. They are "current" simply because you are currently working with them.
2) Save these USER PRESETS with a Relevant Name and date (e.g RhdsFlgr190222... Rhodes Flanger 19Feb2022).
3) Save a MODX Backup to USB. Name the Backup with a Relevant Name and Date. If for example you worked mostly on "Depeche Mode" performances, Name the Backup "DepMode 190222". The Name will later serve as a memory trigger, if you wish to return to any work you were doing in that "period of time".
4) Purge the MODX.... Utility... Reset All Data.
5) Load back your individual user files from Step 1.
6) Optional... Load back ONLY the Libraries you absolutely NEED. Hopefully all the Libraries you own are all stored on your USB, so are only a 2-3 minute load-time away.

This keeps your MODX "Fresh and Current" with lots of room to play with.

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Regarding the FAST EATING of User Waveform Memory.

1) Libraries and User files that come with their own (non-Factory) Samples eat your USER Waveform Memory (1Gb).
2) To see how much User Memory is used by a Library/User Single Performance, there is only 1 labour intensive method I am aware of. But, if that Single Performance is massively important to you, it is worth doing and taking note of the results.

Method
1) Purge your MODX as described above (No Libraries, No User Presets etc).
2) Load in your Library of Choice (e.g. Chick Coreas MkV).
3) IMPORT the Single Performance of choice, from the Library to your USER Bank.
4) Delete the Library
5) Check the Waveform Folder Size (in Mb).
6) You will see the Total Size of the waveforms associated with that single Performance. In this case it maybe 400Mb or so.

Now, you know there is a "high cost" of keeping that Single Performance in your User Bank. Whether you are willing to tolerate that cost is up to you.

Watch out for 3rd Party Libraries that are FULL SAMPLE SETS... these inevitably use a lot of Memory. But they may be worth it to you.

Examples
Bosendorfer
ChickCorea MkV
Hammond B3.

The MODX already has its own FULL SAMPLE SET of a Piano, a Rhodes MkV and a Hammond B3. These cost you Nothing (0Mb). So you must ask yourself... is the Bosendorfer really worth it? The answer maybe YES.... but then you must accept and work within the limits set by the Cost In Memory.

Other things to watch out for...

General 3rd Party "Retail Libraries". An example would be one I bought "Pink Floyd Sound Pack". I was quite dismayed to find that many of the Pink Floyd "Unique" sounds I was seeking, were in fact "One Hit Samples" recorded straight off the Studio Albums (including background collateral... snares, vocal tails etc). These "samples" took up a Massive Amount of Memory, because they were little more than song track excerpts.

But, if I imported a Performance off the library (just to use the Wurlitzer for example)... all the additional Waveform Baggage came with it. Watch out for this... also "Buyer Beware" applies.

By Contrast, the KAPro CS80 has a large library of individual CS80 Oscillator Samples, only a few of which will be used in each Performance. So, you can Import any given Performance without taking a big hit on your User Memory (Import Performance brings only the needed Samples with it, you can then Delete the Library to free up memory, but still have the Samples you "imported").

The CS80 Library as a whole, uses a significant amount of Memory, but individual Performances do not.

Hope this helps.

If you do happen to have a CS80 type library - the OP is likely wishing the task of getting just one Performance into memory would be easier for the benefit of conserving space.

If you happen to have a library that has no significant penalty for importing the whole library (i.e. no benefit for removing all but one Performance) - then this wouldn't matter particularly. Still, it would be "nice" to know where you were without using external tools to determine.

If you have JM Tools then there's some more insight you may gather and possibility to assemble offline.

Without external tools or library documentation that otherwise details the memory impact of every Performance (I haven't seen this yet) - you're left with the extra steps. I can understand why one would want to reduce the back/forth involved (load library, import the keeper(s), generate a new library, load the new library, delete the original library) as after all this you may find no savings as is the case with some libraries. 400MB takes a long time to load over USB2.0 with the extra overhead involved in the transfer (3x in the above process).

Thanks J for mentioning the "minimum penalty" Libraries. To elaborate...

Some 3rd Party Libraries are just Performance creations using the existing MODX/MONTAGE Factory Samples.

I have seen these coined as "Repurposing" Libraries/Performances.

Since the (AWM2) Library Performances are using Factory Samples (Elements) there is no Waveform Memory penalty. The only "hit" is in the "parameter/data config" file size which can be measured in Kilobytes i.e. minimal, like a flat .txt file.

Same applies for Library FM-X performances, which do not use Samples/Elements... so there is no Penalty.

One last tip.... if you didn't already know.

Go to Utility - Contents - Data Utility - Library. Each Library will display its Memory Usage. For example KApro CS80 uses 474Mb. MONTAGE_ Expanded uses 175Mb. etc. You can have up to 8 Libraries Loaded, but only if their Total Memory usage is less than 1Gb.

Ok thanks, Bad Mister. I didn't realize that all the sample data was required for that Performance.

Is it normal for my User library to be 400MB when I only have about 10 MB of custom samples?

Here is how you can review your Waveforms usage:
Press [UTILITY] > "Contents" > "Data Utility" > tap the folder named "Waveforms" - these are in overwritable User memory
If you tap the folder "Library" you can see the memory usage of each - these are in semi-permanent Flash memory

Here is how you can review all the individual Waveforms in your MODX
From the HOME screen:
Press [CATEGORY]
Set the "Bank/Favorite" = All
Set the "Attribute" = All
Under "Main" find "Init"
Select "Init Normal (AWM2)"

Press HOME
Tap the "Type/Name" box for Part 1
A pop-in menu appears > select "Edit"
Along the bottom of the screen tap "Elem1"
Tap "Osc/Tune"

At the top of this screen is the WAVEFORM Name box - if you tap the box it will open a pop-in menu
Select "Category Search"

The amber Waveform Category Search field will appear.
You can, in the upper left corner, select each of the possible Banks that have Waveforms:
Preset, User, followed by your installed Library Waveforms
Review your Waveforms - make sure you do not have duplicates

Panic attack averted. I didn't think I was using anything from the Montage Expanded library so I deleted it.
Several of my performances would no longer play. I reloaded the library and they still didn't work.

I almost puked. Then I noticed the Parts were turned off. Problem solved.

But is there any way to know what dependencies exist between your custom User performances and your libraries?

I'd recommend checking back issues of the Yamaha Music Production Guide.
https://www.musicproductionguide.eu/mpghistory/history_en.htm

There are quite a number of tutorials regarding Library management. The 3 part "Library Guide" series (01-03.2019) is very detailed and also describes using John Melas Waveform Editor to combine Libraries and use only what you need.