GPSMAP 66i - SD card work intermittently

This suggests that removing and reinstalling the card makes a difference.

How far does it get when the boot fails? In particular, do you see the "loading maps" message?

How big is the SD card? I believe the limit for the 66i is 32GB.

Just removing and reinstalling card only in the device doesn't work, the computer step is needed to make it rework.

I tried with differents SD card (all SDHC class 10):
SanDisk High Endurance 32Go
SanDisk Ultra 16Go
Samsung Evo plus 32Go

With the last firmware for the 66i: 9.20

For the boot it fail at the "loading maps" message without displaying the loading bar. And after ~10s it crash (screen go off).

I can make a video if it can help.

Open a support ticket. You've already tried the conventional wisdom things. 

I am experiencing same, but it depends on what maps etc.

Some map sources are very compatible, other crash the device. With some maps, I can keep active only one at time, with other maps I can have 20 and all works.

While 32g is specified by Garmin, a 128 card formated in FAT32 and containing most of the world will do the job too, provided those maps are very compatible.

How to find what is really compatible with 66i? No idea, just try and find out.

If you open a ticket, support will insist on a card that meets the specs - nothing larger than 32GB.

yes, sure, that is why I use 32g cards currently, just to avoid this subject when filing all the crash reports ;-)

On the other hand, in practice, I did not find any differences between 32g card and 128g card.

It is just that my 32g cards I had for the 62s and 64s devices do not work 1:1 in 66i unless I switch only one map (img file) on. Pitty, as I had here the whole world on one 32g card.

Obtaining img from some other sources will make less trouble, but will take more space.

I have heard of issues with 3rd party maps and some sort of internal ID which is embedded in the .img file. But the symptoms don't really match what the OP saw. My recollection is that when 2 maps have the same ID, indexing maps consistently fails. None of this "it works every other time, as long as I sacrifice a chicken on the keyboard between tries"

yes, the maps causing also here those problems have individual ID and work well in 64s for example, where I can activate max 4 img and rename the rest in let say imx or so.

66i refuses to work properly with the same maps, even when only one img is activated. It will crash with 2 maps enabled, it will attempt to start with one  map, but will come into strange state the next time. Sometimes, when I try 5 times, it finally operates again.

Where is the difference, have no  idea so far.

It seem that multiples maps on the SD is not well managed on the 66i.

I tried some different maps like OpenFietsMap, freizeitkarte-osm and multiples different BBBikes extract and they all have the same bug.
Do we have a documentation for .img file?

I'll try to put only one .img map on the SD to see if it still "crash".

Does someone have a ticket opened with the support with this problem?

I dont think Garmin will listen to you if you complain while using maps which are not from garmin.

If the problem persists when using maps from garmin, then you can certainly contact support and if your device is defective it will be replaced.

Clearly, the img files have been analyzed and lot of software is around to build your own maps. Years ago, I was using some Mkgmap and JavaWa software to generate maps.

I remember when I needed to generate a map of Jordan on my w7 computer, it took some 5hrs.

However I am not able to use maps we get from wiki.openstreetmap.org/.../User:Computerteddy on 66i as this will end in symptoms like you describe.

Currently on the 66i I use maps from https://garmin.opentopomap.org/ which are very detailed, but large in volume. They work even when number of files is active.