iMac / Mac Disk Full — Email and Storage Audit

Section: macOS & Apple Hardware Tags: disk-full, storage, mac, imac, email, onedrive, outlook, daisydisk, cleanup Source tickets: #24367 Confidence: High — diagnosis pattern repeated across multiple clients

Symptom

Mac shows "Your disk is almost full" notification, or the user reports the machine is very slow. Storage bar in About This Mac shows the disk at 90–100% capacity. Outlook or other apps may refuse to open or crash on launch due to insufficient temp space.

Root Cause

Common causes in order of frequency:

Outlook email database — Outlook stores a local cache of all email. A mailbox with auto-forwarding rules (e.g., sales@ forwarding to lisa@) results in duplicate messages stored locally, consuming hundreds of GB
OneDrive / Google Drive sync — cloud files synced locally to the drive (especially large file shares)
Downloads folder — accumulated software downloads, installers, attachments
Old iOS backups — iTunes/Finder backups of iPhones/iPads stored locally
Trash not emptied — large files deleted but not purged
Duplicate or orphaned home folders — previous user accounts not removed after staff changes
Application caches — video editing apps, Xcode, VM images

Diagnosis

Step 1 — Run DaisyDisk

Download DaisyDisk from https://daisydiskapp.com (or the App Store)
Open DaisyDisk → Scan the startup volume
Identify the largest consumers — click to drill down into folders
Common findings:

- ~/Library/Group Containers/UBF8T346G9.Office/ — Outlook data (can be 100–500GB+) - ~/Library/CloudStorage/ or ~/OneDrive/ — cloud sync data - ~/Library/Application Support/MobileSync/Backup/ — iOS backups - /Users/<oldusername>/ — orphaned home folder from departed staff

Step 2 — Identify email duplication (Outlook)

If Outlook data is the culprit:

Check whether the mailbox has auto-forward rules: admin.microsoft.com → Users → [user] → Mail → Manage email forwarding
Also check rules in Outlook: Outlook → Tools → Rules — look for rules that forward or copy messages
If sales@ forwards to lisa@: Lisa is receiving two copies of every email — one to her mailbox, one forwarded from sales@. Over years this is catastrophic for storage.

Fix for email duplication: Either remove the forward rule, or configure the forward as a BCC copy in Exchange Admin rather than a full forward (this delivers one copy, not two).

Remediation Steps

Step 1 — Quick wins (do first, immediate relief)

Empty Trash: Right-click Trash → Empty Trash
Clear Downloads folder: Move to an external drive or delete confirmed-unnecessary items
Delete old iOS backups: Finder → [iPhone/iPad] sidebar → Manage Backups → Delete old ones
Remove orphaned home folders: If a previous employee's home folder exists under /Users/, confirm with the client it's safe to remove, then delete it

Expected recovery: 5–30GB typically from these steps alone.

Step 2 — Outlook data reduction

In Outlook: Outlook → Preferences → General → Empty Cache — this removes the local message cache; Outlook re-downloads from the server as needed (safe — all email remains on the server)
If the mailbox itself is enormous (>50GB): recommend Outlook Online Archive:

- In admin.microsoft.com → Users → [user] → Mail → Mailbox archive → Enable archive - Items older than 2 years move to the archive automatically

For duplicate email caused by forwarding: fix the forward rule (see Diagnosis Step 2 above) and compact the Outlook database: Outlook → Tools → Compact Database

Step 3 — OneDrive / Cloud sync

Check OneDrive Settings → Account → Choose folders — only sync folders the user actively needs on this machine
Use Files On-Demand (macOS OneDrive app): right-click synced folders → Free Up Space — files remain in OneDrive but are downloaded only when opened

Step 4 — Long-term: external storage or SSD upgrade

If the internal drive is simply too small for the client's data volume (common on older iMacs with 256GB or 512GB SSDs):

Attach a USB-C external SSD for overflow storage (Samsung T7, 1TB ≈ £60)
For iMacs with HDD: see the iMac HDD → SSD Upgrade article
Move large project archives to the Synology NAS or OneDrive/SharePoint

Verification

After cleanup: About This Mac → Storage — confirm at least 15–20% of disk is free (minimum 15GB). Outlook should open normally and Outlook database health should be good (Outlook → Tools → Database Utility → Verify).

If This Fails

If DaisyDisk shows space is consumed but cannot identify specific files (can happen with APFS snapshots):

Open Terminal and run:

tmutil listlocalsnapshotdates
   tmutil deletelocalsnapshots /

This deletes old Time Machine local snapshots which APFS stores invisibly.

If still unexplained: check for Spotlight indexing issues:

sudo mdutil -E /

(Re-indexes — can free space consumed by a corrupt index.)