External Drive Problems: Slow, Not Recognised, or Freezing the Machine

External drives fail in three characteristic ways, and the cause is usually the connection, the power or the file system rather than the drive itself.

The drive appears in Device Manager but not in Explorer

The system sees the hardware and cannot present it as a usable drive. Which narrows the cause considerably.

No drive letter assigned. Common, and fixed in Disk Management: find the drive, right-click the partition, assign a letter.

Drive letter conflict. The drive was assigned a letter already in use. Same fix, different letter.

Unallocated or unformatted. A new drive, or one whose partition table is damaged, shows as unallocated. Formatting makes it usable and erases everything — so if the drive held anything you need, stop and attempt recovery first.

A file system the machine cannot read. A drive formatted on a Mac may not be readable on Windows, and vice versa. See the file system section below.

A partition marked as inactive or hidden.

Where to look: Disk Management on Windows, Disk Utility on macOS. These show drives that Explorer or Finder will not, and they tell you which of the above applies.

The drive is not detected at all

Try a different cable. Cables fail more often than drives, and the symptom is identical.

Try a different port. Front panel ports are frequently underpowered. Use one on the back of a desktop, connected directly to the motherboard.

Remove any hub. An unpowered hub cannot supply enough current for a mechanical drive.

Listen to it. A mechanical drive should spin up quietly. Clicking, repeated spin-up-and-stop, or silence when it should be spinning are hardware signs.

Try another computer. This separates a drive fault from a machine fault definitively, and it is worth doing before anything else.

The drive slows the whole machine down

Frustrating because it affects everything, not just the drive.

Indexing. After connecting a large drive, the system indexes its contents for search. This is heavy, it runs in the background, and it finishes eventually. If it never finishes, exclude the drive from indexing.

Antivirus scanning the whole drive on connection, with the same pattern.

The drive is failing. A drive with bad sectors makes the system wait on reads that time out, and the whole machine stalls while it waits. This is the most important cause to rule out, because it means data loss is coming.

Check drive health with a SMART reporting tool. Reallocated sectors, pending sectors and read errors are the fields that matter.

A slow connection. An older USB standard, or a drive connected through a port or cable that limits it. A large transfer over a slow connection saturates it.

Power saving spinning the drive down and requiring it to spin up again on each access, producing a pause every time.

Backup or sync software treating the drive as a source and continuously reading it.

The drive freezes the computer

More severe, and it usually points at one of three things.

A failing drive, as above. Reads that never complete lock the system while it waits.

Insufficient power. A mechanical drive drawing more than the port supplies causes it to drop out mid-operation, and the system hangs waiting for a device that vanished. Use a rear port, a powered hub, or a drive with its own power supply.

Driver or controller problem, particularly with older USB controllers and adapters. Try a different port group — front and rear ports frequently use different controllers.

Do not repeatedly disconnect and reconnect while it is hanging. Wait for the system to time out; forcing it mid-write is how file systems get corrupted.

File systems, which cause more problems than anything else

Format	Windows	macOS	Notes
exFAT	Read/write	Read/write	Best for drives moved between systems
NTFS	Read/write	Read only	Windows default
FAT32	Read/write	Read/write	4 GB file size limit
APFS / HFS+	Not natively	Read/write	Mac formats

exFAT for anything shared between platforms.

FAT32's 4 GB limit is why a large video file will not copy to a drive that otherwise works.

Reformatting erases everything. Copy anything you need first.

Before you conclude the drive is dead

Check the health data. SMART reporting tells you whether the drive knows it is failing.

Try it on another machine, with another cable.

Check whether it is the enclosure rather than the drive. External drives are a bare drive in a case with a controller, and the controller fails more often than the drive. Removing the drive and connecting it directly, or in another enclosure, recovers data in many cases where the drive itself is fine.

If it is clicking, stop. Repeated power cycling of a mechanically failing drive reduces the chance of recovery. If the data matters, a recovery service is the option.

Preventing the problem

Eject properly before unplugging, particularly on mechanical drives.

Do not move a mechanical drive while it is running. The heads are close to the platter and shock is what kills them.

Keep it cool and ventilated.

Do not treat one external drive as a backup. A single copy on a drive that can fail is not a backup. See backing up your data.

Check health periodically on any drive holding something you would miss.

The short version

Visible in Device Manager but not Explorer means a drive letter, partition or file system problem — go to Disk Management.

Try a different cable and a rear port first. Cables and underpowered front ports account for a large share of "dead" drives.

A drive that slows or freezes the whole machine may be failing — check the health data before assuming it is software.

exFAT for anything shared between Windows and Mac, and FAT32's 4 GB limit is why large files will not copy.

And the enclosure fails more often than the drive — worth testing before writing off the data.

For current device-specific setup and troubleshooting details, see Apple Support.