The error: Time Machine reports the backup disk isn't available, despite the destination (a network share, an external drive, or a Time Capsule-style device) appearing to be online and reachable.
Environment: Any current macOS version, network or direct-attached Time Machine destination.
Why this happens: This message covers several genuinely different underlying causes lumped into one generic error — a permissions problem on the destination share, a corrupted backup sparse bundle, a network destination that dropped its SMB/AFP connection silently, or (for network destinations specifically) the backup disk image simply being locked by another process that didn't release it cleanly after a previous interrupted backup.
The fix:
- For network destinations, confirm you can still manually connect to the share in Finder (Cmd+K) — if that fails too, the problem is network/share-level, not Time Machine specifically.
- Check Console.app, filtering for "backupd," for the actual underlying error — it's almost always more specific than what's shown in the Time Machine UI itself.
- For a network-based sparse bundle backup specifically, try mounting the .sparsebundle manually via Disk Utility — if it reports the image as already in use or locked, that's very likely your actual cause.
- As a last resort for a corrupted backup destination, starting a fresh backup (after confirming you still have an older backup as fallback) is often faster than trying to repair a badly corrupted sparse bundle.
One thing worth knowing: checking Console.app first, rather than trying fixes based on the generic UI message alone, consistently saves the most time here — the real cause is almost always logged, just not surfaced in the dialog box.