Unable to Install Map Update on Nuvi 2599

That isn't the case. If the map cannot be installed on your device Express should add it to the sd card. You could try the clone method, that should work for you. See www.poi-factory.com/.../39231

I started through that procedure, but encountered what (at least for my setup) is a contradictory step: "Leave the SD card in the computer's card reader and safely remove/eject/unplug your Garmin device from the computer." My desktop can only read the SD card *through* the device. If I unplug the device, the SD drive/reader goes away. Does that mean I have to use a thumb drive instead of the SD card to perform the procedure, then copy the contents of the thumb drive to the SD card? 

I had mentioned earlier that microSD cards, which is what your 2599 uses, come with an adapter that enables it to be inserted into an SD slot.  You further said the Windows 11 laptop had an SD slot.  Did you try to find the adapter?  Another thing you might consider, if you live near a Best Buy, is a new 32GB PNY microSD card is $7 and it comes with an adapter.  You could use the new card, thus eliminating a diagnostic variable and you have the adapter if need be for work on the laptop.  See https://www.bestbuy.com/site/pny-32gb-microsdhc-uhs-i-memory-card/6327962.p?skuId=6327962 for the details on the card.  At the bottom you’ll see a picture of the adapter.

Actually, the only way I accessed the SD card on the laptop was by plugging the Nuvi into a USB port. But now that you mention it, I see a couple of slots in the side of the laptop that I never really noticed before: a small one that's too narrow for my SD card, but also a larger one that's too wide for the card. Now I just need to comb through my stuff to see if there's some sort of adapter that would fit into either slot.  <Later>  OK, I found the appropriate adapter.  But when I run Express, it does not acknowledge the card as the Nuvi or even as a drive. Windows Explorer acknowledges the drive, but does not see the Garmin folder or the garmindevice.xml file; in fact, it says the card is completely empty. When I put the SD card back in the Nuvi and plug the Nuvi into the laptop, the folder and xml file both magically reappear on the SD card. Seems as if there's a missing step in the procedure -- like I'm supposed to make a change in Settings that will allow Express (and Explorer) to see the folder and file on the SD card. 

What is the make and model of the laptop.  Dell for instance often has an SD slot on the right hand side.  You still need that adapter.  It is the same size as an SD card, 32mm x 24mm or about 1-1/4 inch by 1 inch and will have a tiny slot where the microSD card is inserted.

The laptop is HP 14-dq1. The SD slot is on the left side, and as mentioned, Explorer acknowledges the SD card, but not the folder and file. Express acts as if there's nothing in the slot. Apparently the system is acknowledging only the adapter, but not the microSD card inserted into it. There's only one way the adapter and SD card can fit, so I don't think they're inserted incorrectly. Then again, if I do a Properties on the insert, Explorer accurately says the SD card is using 328 MB of space, leaving 14.8 GB of free space; so it seems as if the "phantom" xml file is taking up some space on the SD card -- unless that space is just devoted to the formatting essentials.

Let's make sure all the bases are covered as far as what you see.  Certain files are "hidden" in Windows.  Making sure you can see them is essential.  Do the following...

Change settings in File Explorer to show hidden files

Alternatively, if the method isn't working, you could change File Explorer settings to show hidden files on SD card on phone.

Step 1: Connect the memory card or SD card to your system.

Step 2: Open Windows Explorer, go to Tools and click on Folder Options to View Tab.

Step 3: Now check on Show hidden files, folders & drives. If there is an option that says Hide protected operating system, such should be unchecked.

Step 4: Click on “OK” for all the changes to take effect.

In Windows 11, the steps might be slightly different but basically this should help.

• Press the three dots in File Explorer to open it.  These should be found in the top menu.
• Choose 'Options'.
• Select 'View'.
• Check the box next to "Show hidden files, folders, and drives" under 'Advanced settings'.
• Select 'Apply'.
• Press 'OK'.

Show Hidden Items was already checked. Explorer still seems confused about the "existence" of the file. I'm going to try to create the folder and file again. Maybe the 2nd time it will take.   *** SUCCESS! ***  Express says, "We've got it from here!"  Guess I shouldn't count my chickens before they hatch, but I certainly appreciate all the help!   <Later> The Map Update completed in about 20 minutes. The SD card now shows just over 8 GB in used space and just under that in free space. When I start up Express, it still says Map Update Available, and if I click on Install, it gives me the insufficient memory message again, but now it makes sense. If 8+ GB memory is in use, there's not enough free space for 8 more GB. But now I wonder, going forward, whenever a map update becomes available, will I have to remove what's on the SD in order to make room for the update? Or maybe now that the update has completed for All North America, if I change to All U.S. or Eastern U.S., memory constraints will no longer be an issue.  <Still Later> Guess I spoke too soon. Express still thinks I need to install the update for All North America, won't let me change maps till I install the update, and of course won't let the install go forward because of insufficient memory. Somehow I have to get it to acknowledge that the install succeeded and that the updated map is on the SD card.

Two things you might want to consider.

1.  If that 16 GB card is your only card it might be wise to invest in another.  I use two, and rotate back and forth.  That means my nuvi 2689 always has a card with a current map, and a card with the prior version.  Another reason is the slot, if like my nuvi 2689, is at the lower left corner.  It doesn't take much to have the card pop out and end up between the seats or elsewhere in the car.  I've had one pop out at a car wash during interior cleaning and then vacuumed up.  No fun.  Right now, as I said earlier, you can get a 2nd card without spending much money.  You likely won't be able to find a 16 GB card but the nuvi 2599 can use a card up to 32 GB.

2.  You might want to go to the path (Windows 10)  C:\ProgramData\Garmin\CoreService\Downloads\Map and see how many maps are there.  Express does not do a good job of cleaning up after itself and leaves the maps behind, each being about 6 GB.  I personally delete all but the last two versions.  Then empty the recycle bin and potentially you can gain a large chunk of storage space on your main machine.