diff --git a/src/backuptask.js b/src/backuptask.js
index d59c6740e..7ce5bd9a1 100644
--- a/src/backuptask.js
+++ b/src/backuptask.js
@@ -53,7 +53,7 @@ async function checkPreconditions(backupConfig, dataLayout) {
 
     // check mount status before uploading
     const status = await backups.ensureMounted();
-    debug(`upload: mount point status is ${JSON.stringify(status)}`);
+    debug(`checkPreconditions: mount point status is ${JSON.stringify(status)}`);
     if (status.state !== 'active') throw new BoxError(BoxError.MOUNT_ERROR, `Backup endpoint is not active: ${status.message}`);
 
     // check availabe size. this requires root for df to work
@@ -61,8 +61,11 @@ async function checkPreconditions(backupConfig, dataLayout) {
     let used = 0;
     for (const localPath of dataLayout.localPaths()) {
         debug(`checkPreconditions: getting disk usage of ${localPath}`);
-        const result = await shell.spawn('du', [ '-Dsb', '--exclude=*.lock', '--exclude=dovecot.list.index.log.*', localPath], {});
-        used += parseInt(result, 10);
+        // du can error when files go missing as it is computing the size. it still prints some size anyway
+        // to match df output in getAvailableSize() we must use disk usage size here and not apparent size
+        const [duError, result] = await safe(shell.spawn('du', [ '--dereference-args', '--summarize', '--block-size=1', '--exclude=*.lock', '--exclude=dovecot.list.index.log.*', localPath], { encoding: 'utf8' }));
+        if (duError) debug(`checkPreconditions: du error for ${localPath}. code: ${duError.code} stderror: ${duError.stderr}`);
+        used += parseInt(duError ? duError.stdout : result, 10);
     }
 
     debug(`checkPreconditions: total required=${used} available=${available}`);
diff --git a/src/storage/filesystem.js b/src/storage/filesystem.js
index 76ecbfcfd..7df81e395 100644
--- a/src/storage/filesystem.js
+++ b/src/storage/filesystem.js
@@ -40,10 +40,10 @@ const assert = require('assert'),
     safe = require('safetydance'),
     shell = require('../shell.js')('filesystem');
 
-// the du call in the function below requires root
 async function getAvailableSize(apiConfig) {
     assert.strictEqual(typeof apiConfig, 'object');
 
+    // note that df returns the disk size (as opposed to the apparent size)
     const [error, dfResult] = await safe(df.file(apiConfig.rootPath));
     if (error) throw new BoxError(BoxError.FS_ERROR, `Error when checking for disk space: ${error.message}`);