When Ubuntu Files can read a document but a particular Snap app cannot, the drive itself may be working correctly. A confined Snap can require a connection that grants access to mounted media. This is an additional boundary beyond the permissions of your Linux account.

Compare the same file

Confirm that the affected application is actually the Snap installation. Record its package name and the document's location. Test the same harmless file in Files and in the app; changing both the file and its location makes the comparison less useful.

In a terminal, run snap connections followed by the installed snap's name. This lists its available interfaces and their connections. Find a removable-media entry. An empty Slot shown as a dash indicates that its plug is not connected. If the app exposes no such plug, a guessed connect command cannot add an undeclared capability.

Canonical documents removable-media access for mounted locations under /media, /run/media and /mnt. The grant covers that class of storage; it is not a permission for only the selected document. Ordinary Unix permissions still apply after the connection is made.

Grant only the intended app access

If this is a trusted app that genuinely needs the access, an administrator can connect the exact plug shown by the inspection. For example, the command pattern is sudo snap connect SNAPNAME:PLUGNAME, with both placeholders replaced by the observed values. Do not paste the placeholder command literally.

Reopen the app and repeat the same file operation. Check reading and, if required, saving a disposable copy. Record the connection and result so that support can distinguish a successful access grant from a remaining application error.

When access is no longer required, the corresponding snap disconnect operation removes the connection. If the test still fails, preserve the evidence and examine the path, filesystem permissions or app support; do not loosen permissions across the whole external drive.

Sources: Snap documentation, Snap documentation.