升级High Sierra后无法挂载外接USB Time Machine硬盘，如何解决？

Hey Rich, let's walk through how to fix this—since your drive works perfectly on Linux, we can rule out hardware failure, so this is definitely a macOS-specific filesystem or compatibility issue. Here are the steps I'd recommend, ordered from least to most invasive:

First, we need to pinpoint exactly which disk/partition we're dealing with. Open Terminal (even in Recovery Mode, you can find this in the Utilities menu) and run:
diskutil list
Look for your Time Machine drive—it'll show up with a label like Time Machine or a generic name, under a main disk identifier (e.g., /dev/disk2). Note both the main disk name and its partition (usually something like /dev/disk2s2, marked with Apple_HFS or APFS type).

Disk Utility's GUI can be finicky with unmountable drives, so let's try a forced mount from the command line. Run this, replacing diskXs2 with your actual partition identifier:
diskutil mount force /dev/diskXs2
If this fails, take note of the error message—it'll tell us whether it's a filesystem corruption issue, permission problem, or something else.

Disk Utility's "First Aid" often skips deeper filesystem issues. Boot into Recovery Mode (hold Command+R during startup), open Terminal, and run the appropriate command for your drive's format:

• For HFS+ (most older Time Machine drives): fsck_hfs -fy /dev/diskXs2
• For APFS: fsck_apfs -y /dev/diskXs2
  The -f flag forces a full check, and -y automatically approves repairs. Let it run to completion—if it fixes errors or reports "The volume [drive name] appears to be OK", try mounting the drive again.

High Sierra had some known issues with older USB controllers (like the one in your 2011 iMac). Try these quick fixes:

• Plug the drive directly into a USB 2.0 port (avoid USB 3.0 ports or hubs, which can cause compatibility glitches)
• If you're using a USB hub, connect the drive straight to your Mac instead
• In regular macOS (if you can boot into it), disable USB power saving that might be interfering:
  sudo pmset -a autopoweroff 0
sudo pmset -a standby 0
  Then restart your Mac and test the drive again.

Since Linux can read the drive, its HFS+ tools might catch corruption that macOS is missing. Here's how:

1. On your Linux machine, install HFS+ utilities (for Debian/Ubuntu: sudo apt install hfsutils hfsprogs)
2. Unmount the drive first: sudo umount /dev/sdX2 (replace sdX2 with your drive's partition)
3. Run a forced check and repair: sudo fsck.hfsplus -f /dev/sdX2
4. Once done, eject the drive safely and plug it back into your Mac—this often fixes filesystem inconsistencies that macOS can't resolve.

If all else fails, you can preserve your data using Linux, then reformat the drive for macOS:

1. From Linux, copy all your Time Machine backup files to another external drive
2. On your Mac, open Disk Utility, select the main drive (not the partition), and erase it with:
   • Format: Mac OS Extended (Journaled)
   • Scheme: GUID Partition Map
3. Copy your backup files back to the newly formatted drive, then open Time Machine preferences and re-select it as your backup disk.

内容的提问来源于stack exchange，提问作者Rich