[Galaxy A3, A5, A7, A8, A9 Original Android Develop] setecastronomy:
So they told me but I beg to disagree. The card is almost 6yo, no IO errors found yet, reading/writing speed maxes out the USB2.0 speed. It's good Samsung PRO. Its predecessor (some AData model) was almost trashed within two years (almost means: no IO errors but so slow in certain regions that it's not fun to use anymore).

And the reason for using ext4 is the storage API madness which has hit Google devs with Android 6.0 or so, using this (for permission management) and a specific user as filesystem owner made the life a lot easier.

And so I tried. tar everything, mkfs to exfat, untar. Results: tried to configure the card as "portable storage" and it worked Kind of. Navi app fails to use it's data folder there, music app (MortPlayer) finds to find the media data there. File manager can browse through the files, though.

When I tried some options with OsmAnd, hopping to configure it use map data from portable memory, it reported something about OS not accepting write permissions (but there was no permission question dialog from the OS!), and that it has to change settings and restart the app. Okay, did so. After the app was restarted, it got stuck for about 10 seconds and then the whole phone suddenly rebooted.

Tried to investigate what happened, and cannot see much, only this:

08-23 19:43:03.737 2275 2275 D vold : exfatprogs version : 1.1.2
08-23 19:43:03.738 2275 2275 D vold : ERROR: /Android/data/net.osmand.plus/files/rendering/default.render.xml: first cluster is wrong. Truncate (y/N)? n
08-23 19:43:03.738 2275 2275 D vold : /dev/block/vold/public:179,33: checking stopped. directories 1256, files 12597
08-23 19:43:03.738 2275 2275 D vold : /dev/block/vold/public:179,33: files corrupted 1, files fixed 0

then some other stuff and after few minutes this:

08-23 19:55:04.231 2479 2494 D installd: Device /data has 129531904 free; requested 1048576000; needed 919044096
08-23 19:55:04.633 2479 2494 D installd: Refusing to clear cached data in reserved space
08-23 19:55:04.634 2479 2494 E installd: Failed to free up 1048576000 on /data; final free space 129531904: Success
08-23 19:55:04.637 2593 2691 W DeviceStorageMonitorService: java.io.IOException: Failed to free 1048576000 on storage device at /data
08-23 19:55:04.637 2593 2691 W DeviceStorageMonitorService: at com.android.server.pm.PackageManagerService.freeStorage(PackageManagerService.java:5200)
08-23 19:55:04.637 2593 2691 W DeviceStorageMonitorService: at com.android.server.storage.DeviceStorageMonitorService.check(DeviceStorageMonitorService.java:193)
08-23 19:55:04.637 2593 2691 W DeviceStorageMonitorService: at com.android.server.storage.DeviceStorageMonitorService.access$100(DeviceStorageMonitorService.java:72)
08-23 19:55:04.637 2593 2691 W DeviceStorageMonitorService: at com.android.server.storage.DeviceStorageMonitorService$1.handleMessage(DeviceStorageMonitorService.java:257)
08-23 19:55:04.637 2593 2691 W DeviceStorageMonitorService: at android.os.Handler.dispatchMessage(Handler.java:106)
08-23 19:55:04.637 2593 2691 W DeviceStorageMonitorService: at android.os.Looper.loop(Looper.java:223)
08-23 19:55:04.637 2593 2691 W DeviceStorageMonitorService: at android.os.HandlerThread.run(HandlerThread.java:67)
--------- beginning of kernel

(and the boot sequence, apparently)

After the reboot, there is the "card setup" wizard coming back, asking again "portable storage" or "internal". Selecting "portable" tells that the card needs to be formatted. WAT? How did it work the first time? I went to the shell, and I can mount the block device just fine and read random files from there.

So now, I am at the brink of formatting it as "internal storage" to have my peace again. Google managed to kill this Android feature, it took a while but they got me.