His Toshiba external stopped opening but did not disappear. It "recently stopped mounting" to his laptop, and though Mac-formatted and tried on both his "personal and work Macbooks", the result is consistent: "the laptops can read the hard drives in disk utility but they wont mount, so i cant open my files." A drive that appears in Disk Utility but refuses to mount, on more than one Mac, has a logical volume fault — not a dead drive — the hardware is being detected and the volume is present, but its filesystem structures are damaged enough that macOS cannot mount it, which is the recoverable end of the spectrum.
| Media | Toshiba StorE alu2 external hard drive, Mac-formatted — visible in Disk Utility on two different Macs but refusing to mount; the owner's files inaccessible. |
| Reported situation | Drive stopped mounting to the laptop · Mac-formatted · tried on both personal and work MacBooks · appears in Disk Utility on both but will not mount · files inaccessible · recovery cost queried. |
| Fault class | Filesystem structure corruption on a detected drive — the volume present but unmountable behind damaged structures, distinct from a hardware failure where the drive would not appear; drive health to be confirmed. |
| Equipment used | Drive imaged under a hardware write-blocker before any interpretation · drive health confirmed on PC-3000 UDMA, since it is detected and the fault appears logical · Mac-native APFS/HFS+ structures parsed from the image in R-Studio, backup copies of the volume structures located and rebuilt · signature carving for anything the damaged structures no longer reference · recovered files validated by rendering. |
Appearing in Disk Utility is the reassuring part, and it rules out the worst cases. That both Macs can see the drive in Disk Utility means it powers, communicates, and is detected — the hardware and the drive's basic electronics are working. A drive that had died mechanically or electronically would not appear at all. Being visible but unmountable is a narrower fault: the drive is fine enough to be seen, but its volume will not open.
Reads-but-won't-mount describes a filesystem macOS cannot parse. To mount a volume, macOS reads the structures that define and describe the filesystem. When those structures are damaged, the volume appears in Disk Utility — because the disk and partition are detected — but will not mount, because the filesystem cannot be made sense of. The data area behind those structures is typically intact; the fault is in the small records that let the system present the volume, not in the space holding the files.
The same result on two Macs confirms it is the drive, not the computer. Testing on both a personal and a work MacBook and getting the identical visible-but-unmountable behaviour eliminates any single machine's configuration or software as the cause. The fault travels with the drive, which is exactly what a logical volume problem does — and it points the work at the drive's filesystem rather than at either Mac.
The move to resist is Disk Utility's repair and erase, which is the main hazard. Faced with an unmountable volume, macOS offers First Aid, which writes repairs to the disk, and ultimately erase. On a volume whose structures are damaged, First Aid can worsen the corruption, and erase would destroy the recoverable data outright. Neither should be run on the original drive; it should be left as-is and recovered from an image.
Recovery reads and rebuilds the structures from a copy. The volume's structures carry backup copies, and rebuilding from those restores the files with their names and organisation. Where structures are missing, signature carving recovers file data directly. Running both and reconciling them returns the maximum from a drive that is fundamentally healthy but logically broken — organised files where possible, carved files elsewhere.
The honest prognosis is good, with drive health confirmed first. A detected drive with a logical volume fault usually recovers well. Drive health is verified at the outset to rule out a hardware fault developing behind the logical symptoms, and the recoverable content is confirmed before any charge — with everything done on a write-blocked image so First Aid never gets the chance to make it worse.
The drive was imaged under a hardware write-blocker before anything interpreted it, and its health confirmed on PC-3000 UDMA. The Mac-native APFS/HFS+ structures were parsed from the image in R-Studio, backup copies of the volume structures located and rebuilt, and signature carving used for anything the damaged structures no longer referenced. The recovered files were validated by rendering.
Detected drive imaged, the volume structures rebuilt from their backup copies, and the files returned with their organisation. The diagnostic costs nothing and completes within two working days of arrival, and the quote is one fixed written figure with VAT already in it; if the data can't be brought back, no charge is made. The decode: appearing in Disk Utility means the drive is alive — visible but unmountable is a damaged volume, not a dead drive. Rebuilt from the volume's own backup structures on an image, your files come back, and First Aid never gets to deepen the damage.
Don't run First Aid and never click erase — First Aid writes to a volume whose structures are already damaged and can deepen the corruption, and erase destroys your data outright. The drive appearing in Disk Utility means it's alive and detected; visible but unmountable is a logical fault with the data intact behind it. Getting the same result on two Macs confirms it's the drive, not the computer. Stop replugging it, and have it imaged and recovered from a copy.
Our case files are written up from genuine enquiries our lab has handled for customers across Swansea and South Wales, anonymised to protect client confidentiality. Each one describes the diagnostic and recovery approach our engineers apply to that fault, using the equipment listed.
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