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Mod installation fails on instances stored on an external SSD on macOS – I/O (std) error: Operation not supported (os error 45) #7578

Description

@notesow

Please confirm the following.

  • I checked the existing issues for duplicate problems
  • I have tried resolving the issue using the support portal
  • I have ensured my Modrinth App installation is up to date

What version of the Modrinth App are you using?

Modrinth App 0.21.3

What operating systems are you seeing the problem on?

MacOS

Describe the bug

I have been experiencing issues with the Modrinth App when using an external SSD on macOS for quite some time, but over the last few days the problem appears to have become more widespread and more severe.

My system is a MacBook Pro with an Apple M1 Pro chip. Due to storage limitations, I use an external SSD to store my Minecraft instances and the environments I also use for mod development.

My current setup is therefore:

Modrinth App → Mac internal drive
Minecraft instances → external SSD
Instance mod folders → external SSD

Initially, the problem only affected the Modrinth App itself when the application was also installed on the external SSD.

Whenever a Modrinth App update was available and I tried to update the client, the application would close and the update would fail with an error.

As a workaround, I moved the Modrinth App to the Mac internal drive, inside the Applications folder, while keeping all Minecraft instances on the external SSD.

With this setup, the issue appeared to be at least partially resolved: the Modrinth App was installed on the internal drive, while the instances remained on the external SSD, and until around two days ago I was still able to install mods normally directly from the app.

Now, however, the issue has also started affecting operations performed on instances stored on the external drive.

When I try to install a mod through the Modrinth App into an instance stored on the external SSD, the following error appears:

I/O (std) error: Operation not supported (os error 45)

Based on the app behavior, the operation initially appears to be considered successful at the UI level, because the mod button changes from "Install" to "Installed".

However, the change is not actually persisted in the instance.

If I then open the list of installed mods, the mod is not present. If I go back to the same mod page in the built-in browser, the button changes back to "Install".

There therefore appears to also be a mismatch between the state temporarily shown by the UI and the actual result of the filesystem operation.

Steps to reproduce:

  1. Use the Modrinth App on macOS with Apple Silicon.
  2. Install the Modrinth App on the Mac internal drive.
  3. Configure a Minecraft instance whose directory is located on an external SSD.
  4. Open the instance through the Modrinth App.
  5. Open the section used to browse and install new content.
  6. Select a compatible mod and click "Install".
  7. The following error appears:

I/O (std) error: Operation not supported (os error 45)

  1. The UI may temporarily show the mod as installed.
  2. After checking the instance, the mod is not actually installed.
  3. Returning to the mod page shows the "Install" button again.

Expected behavior:

The Modrinth App should be able to perform normal operations on instances stored on an external volume, including downloading, creating files, and installing mods.

Instances stored on the external SSD were already working correctly with this same setup until a few days ago.

Actual behavior:

The installation fails during an I/O operation on the external volume with:

Operation not supported (os error 45)

The UI initially shows the installation as completed, but the corresponding mod/file is not actually added to the instance.

Possible regression:

I think it is important to point out that this exact setup was working correctly until around two days ago.

I have not intentionally changed the instance location: the instances are still stored on the same external SSD.

Previously, the issue related to external drives seemed to mainly affect Modrinth App updates when the app itself was installed on the SSD.

Now, however, it also seems to affect filesystem operations performed by the Modrinth App on instances located on an external volume.

It may therefore be useful to verify which specific filesystem operation is triggering os error 45, especially in case a recent change introduced a new method for copying, moving, creating, or finalizing mod files on macOS.

Impact:

For my use case, moving all instances to the Mac internal drive is not a viable solution because there is not enough available storage.

I use the external SSD both for playing Minecraft and for mod development, so support for instances stored on external volumes is important for being able to use the Modrinth App normally.

Steps to reproduce

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Expected behavior

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Additional context

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