Can I concatenate disks (JBOD) without erasing the data on the disks?

I would advise against doing this.

First, in order to create the spanned (concatenated) disks you must first format & erase both disks. You have already discovered this.

Second, even if it works, it increases the risk of failure and loss of all data on both of the "concatenated" drives.

Combining individual stand alone external drives is open to a huge point of failure since drives will go ready at different times during boot. Plus how do you connect them if the computer is already powered on? Will this break the setup? What about accidentally disconnecting the drive? Sometimes just bumping the cable can cause a momentary disconnect. What about waking the Mac and the drives from sleep? I highly recommend you test the various possibilities before using it with important data.

If all of the drives in the JBOD/RAID are inside a single enclosure, then many of these issues are eliminated or at least minimized.

We've had Apple software RAID mirrors break out of the blue for unknown reasons and this was even when the drives were on internal bays in a Mac Pro. When you reconnect the drive to the array it will be reinitialized into the RAID.

Here is an article about the JBOD setup where it mentions about what happens when a drive fails. It does mention that the data is stored on individual drives so recovery should be possible, but it may require more work than just connecting the drive to the computer:

https://www.lifewire.com/use-disk-utility-to-create-a-jbod-raid-array-2260916

That does not mean that recovering data is as simple as mounting a member of the JBOD RAID set and accessing it with the Mac's Finder. You might need to repair the drive and use a disk recovery application.

If this is something which must always work, then you are better off getting a dedicated external enclosure for the JBOD/RAID, or perhaps even a NAS if the transfer rates are sufficient for you. I personally would never combine individual USB drives into a complex array as there is too much to go wrong. But it is your system.

On the fly, disk concatenation, w/o loosing existing data, ... i have not seen any such app for macOS , someone else may provide an answer to such app if such exists for macOS.

as far as i remember,

in windows OS, there were apps for such purpose,

and some RAID controller card's embedded firmware interface supported such functions.

In those, one could add new (or 2nd) physical disk, & that 2nd/new disk's GB/TB storage size could be easily added into an existing JBOD volume : by selecting the new/2nd drive, & then by selecting the option such as "Span/Expand volume into all selected disks".

In windows OS or in embedded interface, this function usually formatted the new/2nd disk, and warning was also shown that new/2nd disk's all data will be lost.

Existing volume's earlier disk's data was not affected.

And this used to work fine for non-encrypted volumes.

Encrypted volumes could not be handled in such way, or some other limitations were present.

When newer macOS is running, then it creates lots of obstacles purposefully, to not-allow full disk control/usage by other apps,

so, such functionality is harder to do in newer macOS & inside a running OS,

unless apple itself approves & allow it to some software devs.

But i think,

If you open up your main apple computer (please do not do this if you do not want to loose warranty : opening up hardware will cancel your warranty),

& move the (original volume's) storage disk into an external USB-to-SATA/M2 etc based adapter/enclosure,

And also place the 2nd/new disk into an external USB-to-SATA/M2 etc based adapter/enclosure,

Then,

You may be able to use another/2nd apple computer,

to select multiple disks and Expand/Span existing volume into the new/2nd disks, w/o loosing existing data.

But if that software/tool shows warning (in 2nd apple computer), that, your 1st disk or main/original volume disk's all data will be erased/lost, then you should stop right there.

And, before doing any of these : make a CLONED copy of original volume disk, if possible . (Not other type of copy).

If you make mistake w/o understanding a command / button / prompt , then your original volume data will be lost.

And data recovery will not be able to give back 100% same file-system-structure as before.

But if the software / tool in 2nd apple computer , do allow to Expand/Span the existing main Volume into the new/2nd disk,

then, you just have to put the main/original volume disk back into your main apple computer,

and add the new/2nd disk via same USB-to-SATA/M2 adapter/enclosure, into your main apple computer.

Another option : get RAID controller card that supports apple's new macOS's new file-system format , ( this is not-easy to find ),

then use that RAID card on a PC or MacPro computer , to expand existing volume , into multiple disks , w/o destroying 1st disk's existing volume data.

But best is,

find a solution that allows you to boot your main apple computer,

by using an external (or internal) bootable disk or drive , etc,

Which has a bootable OS, & necessary software.

Then you can plugin new/2nd disk into your main apple computer by using USB-to-SATA/M2 adapter/enclosure,

Then run the RAID software or Volume/partition handling software.

In this way, all hardware components & sub-component trees & IDs , etc remain same,

So, when this process is successful, then you can just easily boot back from original volume.

Does the user actually mean Clone one drive External Drive to another drive External Drive ?

That can be done provide the first External Drive is Not a Time Machine Drive

This can be done using a Third Party software I have used for may years.

Have even using it to Clone a 2TB External Drive with all my Videos to 8 TB External Drive,

https://bombich.com/features

Thank but think your reply was intended for the Original Contributor who responded to your question.

As I had mentioned. CCC can be use but not if the Original Drive was used for ime Machine Backups

Refer to below from the Developer of CCC

Can I use CCC to copy a Time Machine backup?

Excerpt for above link

No. Copying a Time Machine backup volume with anything other than the Finder is not supported (by us, nor Apple); CCC specifically disallows copying anything to or from a Time Machine backup volume. Apple does not document a procedure for making copies of Time Machine volumes.

larry666 wrote:

I appreciate that a JBOD "RAID" isn't actually a proper RAID, in that it provides no data redundancy. I already have a NAS RAID for that :-)

I was only pointing out that the JBOD setup would have the same issues as a software RAID mirror.

The JBOD "RAID" I have set up is to expand the capacity of an external volume by combining an existing 3

The alternative would be to buy a larger external drive but I'd prefer not to spend the money for a 7 TB drive when I have 4 TB drives sat on the shelf.

I can certainly appreciate wanting to re-use what is already on hand. Just as long as you are aware of the risks and are willing to deal them. A lot of people come here with unnecessarily complex setups frustrated & complaining about them wondering why they are having problems & needing desperate immediate help to recover data that was not backed up. Just wanted to help prevent that from happening to you and anyone else who comes across this thread.

I hope this setup works out for you.