I checked only the read and write. I did not check FORMAT
I will check.

Here you can find the possible options and defaults.

Also, in this page it is explained the three working modes of the firmware, the HxC mode left as a legacy mode,  for Amiga, Atari ST, CPC and others.

What I want to know: how change the mounted images from the host computer?

Probably you put a HXCSDFE.CFG then it is can be used?

What I want to know: how change the mounted images from the host computer?

Here's the Manual:

https://hxc2001.com/download/floppy_drive_emulator/SDCard_HxC_Floppy_Emulator_Direct_Access_mode.pdf

You "move" the "head" to track 255, this switches the HxC to a special mode.
"Sector" 0 on "Track" 255 will then give you access to a status/control sector you can read and write to. Use the "struct direct_access_cmd_sector_" to write command 4 (SELECT_IMAGE_INDEX) with an appropriate image index.
Wait for one index pulse from the disk, then read sector 0 again to verify your command has been executed.

Once you "move" the "head" back to track 0, the image should be selected.

Using other commands, you can even achieve full LBA access to the complete SD card.

Tobias

Is 353 KB free the correct result ?

Only if you use 360K drive
Which drive used?

I already found bug(?) in EXDOS, when checking SD/DD. But it is usually at old drives with head settle mechanism. Some of these drives don't execute stepping until the head loaded. Then the first few step in command lost, and when counting back the tracks got a false result, then detect as SD drive.
My trick for these drive: :DIR, then write the :FORMAT command under the results, go back with the cursor, press ENTER again on the DIR and at the end immediately and quickly press ENTER on FORMAT, and Yes on the question. If you do it before the motor stopped, then the FORMAT started on the fly, and the drive detected correctly.

In the FAFO I handled this situation (do one read before SD/DD detection). This is the one of the reasons why I started write my own formating program.

This is a 3.5 "DD / HD drive with a DD diskette.

Which model? I will try to check same.

With the trick works better - 712 KB free.

I use FAFO25, this works always flawless without tricking.

The EXDOS Compact card without the WD1772 chip costs 49 EUR.

Can I ask a prices without ROM chip and 1772? And full complete with 256K Flash and 1772?