Cannot read fat32 external drives: Couldn’t modify partition map. : (-69874)

It appears the drive is write protected. Some will have a small write protect switch that you can toggle. If yours does not have a write protected switch, then search the web for "Sandisk cruzer write protected" for suggestions on disabling it. This is a common problem with the SanDisk drives and there are many different solutions available for you to try. Users have not found a single solution to solve the problem. For some, simply starting in Safe Mode worked.

Use safe mode on your Mac - Apple Support

What if you Get Info on the "DOS_FAT_32" volume which is "disk4s1" at the time? You only provided the info for the physical drive.

Also try updating to 13.6.1 (or 13.6.2 depending on the model of the Mac).

Did you erase the whole physical drive or just the existing volume? 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. Try erasing the physical drive as GUID partition and FAT32 (or ms-dos), or MacOS Extended (Journaled) just for testing purposes to see if either the GUID partition type makes a difference or the file system. If you need the legacy MBR partition type, then try using that again.

Sometimes macOS & Disk Utility can get confused by the smallest thing when it comes to drives, especially a USB stick. If you are familiar with the command line, then write zeroes to the beginning of the USB stick to overwrite the partition table & start of the file system. You don't need to write too many zeroes....maybe 500MB although I usually end up writing about a 1GB of zeroes just to be safe.

Is this one of the USB stick models which may have a write disable switch or feature? I know you have it working sometimes, but maybe macOS is getting hung up on that feature while Windows is not.

liriarte wrote:

If I am not wrong I'd need to run the following

sudo dd if=/dev/zero of=/dev/disk4 bs=500M

Yes, assuming the drive you want to erase has the drive identifier of "disk4" and is unmounted (different than ejecting it). Except for macOS you must use a lower case letter for the size designation "bs=500m". However, this does not write just 500MB, but indicates a block size of 500MB.

All volumes on the drive being erased must first be unmounted. You can use Disk Utility to unmount all volumes on the drive. Ejecting the volumes from the Finder does more than just unmount the volumes, it also disconnects the drive from the system so it is no longer accessible to the OS. It can also be done from the command line. Assuming the drive has the identifier "disk4", the following command can be used:

disktutil  unmountDisk  disk4

but from Recovery Mode and after csrutil disable. Does it make sense?

No need. You are erasing an external drive. It can be done while booted normally into macOS. Recovery Mode is only needed if you are erasing the internal boot drive. Even then I don't think you need to disable SIP since you would be operating on the whole physical drive.

On the other I've managed to read/write/erase on my other Mac (Monterrey 12.2.1), so this seems to be definitely something with this Mac (Ventura 13.6.2)

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

What third party software or configuration of macOS differs between them?

Perhaps other connected devices? Disconnect all other devices to see if that makes a difference.

If you are using any anti-virus apps, cleaning/optimizer apps, or third party security software, then uninstall them by following the developer's instructions since these types of apps are not needed on a Mac and usually causes more problems than they solve. You can run EtreCheck and post the report here so we can examine it for possible clues.

I did have a USB stick where it seemed to work fine on one Mac, but refused to work on another Mac. If I zeroed the beginning of the USB stick, then the other Mac was able to erase it and use it & it seemed Ok, but then the first Mac could no longer access the USB stick. It ended up being a bad USB stick. I had never before encountered such behavior before.

liriarte wrote:

I can try writing zeroes. Shall I do that and erase afterwards?

Yes. When you write zeroes to the beginning of the drive you destroy the partition table so the drive should appear completely blank. If the USB stick has no hardware issues, then this should work. Otherwise, I would suspect a hardware issue of some sort.

Do you know how to use the command line to write zeroes to the beginning of the USB stick or do you need instructions? How well do you know the command line?

liriarte wrote:

I erased the drive from my other Mac with Monterrey 12.2.1, but didn't make a difference.

Just an aside, Monterey is up to 12.7.1. You should update.