diff --git a/backend/protondrive/protondrive.go b/backend/protondrive/protondrive.go
index 9188383cf..4595ea5dc 100644
--- a/backend/protondrive/protondrive.go
+++ b/backend/protondrive/protondrive.go
@@ -264,9 +264,14 @@ func shouldRetry(ctx context.Context, err error) (bool, error) {
 	}
 	var apiErr *proton.APIError
 	if errors.As(err, &apiErr) {
-		// Transient storage block error ("Please retry").
-		if apiErr.Code == 200501 {
-			fs.Debugf(nil, "Retrying storage block error: %v", err)
+		// Code 200501 is a generic Drive operation-failure code. Proton also
+		// returns it with an HTTP 422 for permanent validation failures (for
+		// example a content key packet that cannot be verified, or an upload
+		// format the account is not enabled for), which must NOT be retried -
+		// doing so spins the pacer until the operation times out. Only retry it
+		// when it is not a permanent client (4xx) error.
+		if apiErr.Code == 200501 && (apiErr.Status < 400 || apiErr.Status >= 500) {
+			fs.Debugf(nil, "Retrying transient storage block error: %v", err)
 			return true, err
 		}
 		// Server errors. 503 is already handled by the SDK; retry the rest.
diff --git a/backend/protondrive/protondrive_internal_test.go b/backend/protondrive/protondrive_internal_test.go
index aaf2945ae..d1ef4224d 100644
--- a/backend/protondrive/protondrive_internal_test.go
+++ b/backend/protondrive/protondrive_internal_test.go
@@ -77,7 +77,8 @@ func TestShouldRetry(t *testing.T) {
 	}{
 		{"nil error", ctx, nil, false},
 		{"cancelled context", cancelledCtx, errors.New("some error"), false},
-		{"storage block error Code=200501", ctx, apiErr(422, 200501), true},
+		{"permanent validation error Code=200501 Status=422 (not retried)", ctx, apiErr(422, 200501), false},
+		{"transient storage block error Code=200501 Status=500 (retried)", ctx, apiErr(500, 200501), true},
 		{"server error Status=500", ctx, apiErr(500, 0), true},
 		{"server error Status=502", ctx, apiErr(502, 0), true},
 		{"server error Status=504", ctx, apiErr(504, 0), true},