External SSD not working

Make sure to connect the drives directly to the computer and try connecting just a single drive for testing purposes. Also disconnect all other external devices in case one of them is causing a problem.

When erasing the drive, make sure to erase the whole physical drive. Within Disk Utility you may need to click "View" and select "Show All Devices" before the physical drives appear on the left pane of Disk Utility.

What are you doing with these drives after you have erased them? Are you able to erase them using Disk Utility? Are you able to save a file to the external drive after it has been erased and before doing anything else to the external drive? Have you tried saving a newly created file to the external drive immediately after erasing the external drive?

For the Sandisk SSD, check to see if you have a model which requires a critical SSD firmware update to resolve a data loss issue (the data loss part is not noted in the WD/Sandisk support page):

https://support-en.wd.com/app/firmwareupdate

Also see the following articles regarding recent issues with some of the Sandisk external SSDs:

https://arstechnica.com/gadgets/2023/05/sandisk-extreme-ssds-keep-abruptly-failing-firmware-fix-for-only-some-promised/

https://arstechnica.com/gadgets/2023/08/lawsuit-takes-western-digital-to-task-over-sandisk-ssds-allegedly-erasing-data/

Try booting into Safe Mode to see if that makes any difference.

dsattta wrote:

Are you able to save a file to the external drive after it has been erased and before doing anything else to the external drive? I can only copy and paste separate files, not folders. So, e.g., if I drag'n'drop whole folder with pictures, I get the error message, if I copy only the photos (or other files, like music), I am able to do that. I tried the dot_clean command in terminal, it also did not help.

Have you tried saving a newly created file to the external drive immediately after erasing the external drive? Yes, same issue. I could only copy separate files, not folders.

I'm not entirely clear by some of this. I also may not have been very clear either when I asked if you could "Save" a file to the external drive immediately after erasing the external drive. I meant for you to create a new file in an app like TextEdit, then when saving the file, selecting the external drive as the destination. Do not try to save to the internal drive first as it may possibly corrupt the file somehow. Also, using an Apple built-in app like TextEdit is best since it minimizes the chance of a problem with a third party app & their settings.

So launch Text Edit. Type anything into document you want. Then when you click "File --> Save...." you will want to select the volume on the external drive & click the Save button (default name will be "Untitled.rtf").

If this works, then the next test would be to use the Finder to create a new folder on the external drive. If that also works, then try copying the file you created with Text Edit into the new folder. If you hold the Option key while dragging the file to the new folder it should make a copy (should show a "+" symbol by the mouse pointer).

If this also works, then try moving the file to your internal drive within your home folder (Desktop, Documents, Downloads, or even the home folder itself). Then try copying the file from your home folder to the external drive again.

Edit: Are you using iCloud? I have seen a couple people mention that turning iCloud off & back on resolved the -36 error message.

Did you try first aid on the drives?

My guess is the drive is faulty.