What gets moved
Everything in the runtime path: instances (with their mods, worlds, configs), Minecraft's shared assets, Minecraft's shared libraries, Java installations downloaded by GDLauncher, the launcher database, and the app-wide logs. This is essentially everything heavy GDLauncher created on your machine.
What doesn't move: the app data path (window state, Electron cache, top-level config). That stays in its OS-standard location; you usually never touch it.
What size to expect
A clean install with no instances is around 1 GB (mostly the Minecraft assets and a Java install). Each modpack adds roughly its own download size, so a few large packs can push it past 50 GB. Check your current data path's size in your OS file manager before moving to estimate how long migration will take.
If you've already moved data manually
If you copied gdlauncher_carbon/data to another location by hand (without going through Settings), you can point GDLauncher at the new location. When you type a path that already contains valid GDLauncher data, the migration is skipped, GDLauncher just updates its config to point there. Useful for moving to a different machine and then pointing a fresh install at the migrated data.
If migration fails
The overlay turns red and shows the error. GDLauncher rolls back the files it copied to the new path, keeps the marker pointing at the old location, so you can retry without losing anything. Most common causes:
- Out of disk space on the target. The target drive needs at least as much free space as the source uses.
- Permission denied. On Windows, some folders (Program Files, the root of C:\) need admin rights. On Linux/macOS, the target must be in a path you own. Pick a user-owned folder.
- Target is read-only. Some network drives mount read-only by default. Check that you can create files there outside GDLauncher first.
App-wide logs at <runtime_path>/__gdl_logs__/ have more detail if you need it.
The runtime_path_override marker
After a successful move, GDLauncher writes a small text file named runtime_path_override inside the app data path (not the runtime path). The file contains the new runtime path; every launch reads it to know where data lives. If you delete it by accident, the launcher falls back to its default location and won't see your moved data until you go to Settings → Runtime Path and point it back at the folder you moved to (it'll register as "already in use" and switch to it without copying).
Don't sync the runtime path while playing
OneDrive, iCloud, Dropbox, etc. compete with GDLauncher for file handles. If the runtime path is in a folder these tools actively sync, you'll see weird "file in use" errors and occasional save corruption. Put the runtime path on a drive that's only backed up offline (e.g. by snapshots, restic, Backblaze), not one that's continuously syncing.