I got a machine with MX Linux as an only OS (for now) already run. I want to add an extra HDD for AROS there. My goal is to have a single machine with two separate hard disks. One with MX Linux, and the second with AROS.
My question is what filesystem to choose for an extra partition to easily exchange data between AROS and Linux?
The MX Linux HDD has extra space left for future partition. HDD for AROS is blank for now.

Linux should support FAT32 and NTFS partitions that could be shared with AROS, FAT32 being the best choice.

Unfortunately, looks like AROS can't see the second HDD.
It can see only the Linux hard drive (unit 0) and (unit 5) the DVD drive.
Maybe it's because the drive intended for AROS is ab IDE HDD conected via an SATA<->IDE adapter?
Linux can see it and can format it.

Is the second HDD formatted FAT 32?

No. It's plain. I deleted all existing partitions under Linux before starting AROS installation.

If you create a FAT32 partition, it is likely that AROS will handle it

Try changing the SATA Controller in the BIOS to IDE mode.

I have windows 7 in dual boot the ntfs partitions are visible and you can operate on them thanks to the ntfs3g-handler

There is also ntfs-handler but it is read-only, as mentioned I recommend using FAT32 which tested is more secure.

AMIGASYSTEM is right, FAT32 the best filesystem to exchange files between AROS and other OSes. Don't use exFAT, make sure it is FAT32.
I actually use a FAT32 formatted 2GB USB drive. Works very well for what you are trying to do and from computer to computer.

never used exFat just that the partitions I have visible on wanderer are in ntfs and the ntfs3g handler behaves well, then the usb sticks I have all in fat32

ntfs-handler is read only, ntf3g-handler is read and write and works fine

Amiwell as I said NTFS still needs to be well tested !!!

- If you use an NTFS formatted Pendrive, but I suppose also HD, it often happens that you cannot delete a folder that includes files or folders.

- An empty folder in some cases you have to repeat the delete command several times to delete it.

- If you boot AROS with an NTFS-formatted Pendrive inserted in the USB port, i would receive a request for volume 'L", consequently AROS will boot without Thema windows.

I recommend not using ntfs-handler unless you are an expert if you use a DualBoot with Windows, or if you have a shared Windows system, I say this because by distraction you might delete Windows system files.

First of all thank you all for your support.

So, If I first create two fat32 partitions for AROS, should I choose "Use existing Aros partitions" option?

If you're talking about installing AROS no, the partition where AROS is to be installed must be "Raw" uninitialised and unformatted, whereas you can initialise and FAT32 format the second partition which you just need to share with AROS.

If InstallAROS encounters a HD with two FAT32 partitions it will destroy them and create a single partition where it will then install AROS!