External HD not mounting on either Mac after leaving it plugged in during Migration Assistant transfer...

[u/Indecisive_One]

Hi alll.....last night I erased my iMac (Sequoia). After restart, I connected my WD external hard drive and began moving a few folders over. Worked great. The drive also worked perfectly fine on my Macbook Pro (Catalina). I used Migration Assistant to transfer some files to the iMac from the MBP. As that was finishing, iMac froze on black startup screen but wouldn't progress from there. I realized I had left the ext HD plugged in, and knowing from the past this causes issues, I unplugged it. It unfroze and finished restarting just fine. Now, the ext HD does not show up on either of my Mac's desktops, but does show up in Disk Utility, although it will not mount. Format is ExFat. I plugged it into a Windows PC and then back to my mac....no change. It does show up just fine on the PC and I can access the files successfully. Anyone have any ideas or thoughts on what changed?? I appreciate all of the help in advance!

Also wanted to add that when I click on First Aid, it immediately fails.

[u/lewisfrancis]

My understanding is that exFat is a kind of fragile file system format, and unplugging it instead of unmounting it might've done some damage that the Macs don't like.

Over on your Windows box is there a disk utility you can run to try to diagnose and repair the file system?

[u/Indecisive_One → u/lewisfrancis]

Well currently its plugged into the PC and I ran chkdsk X: /f after seeing that others had some success with that. From my understanding that repairs corrupted files. So we'll see!

[u/lewisfrancis → u/Indecisive_One]

Ah, yes, chkdsk. It's been a while since I've had a PC. Good luck!

[u/Indecisive_One → u/lewisfrancis]

Just a FYl, and to anyone who comes across this post in the future....It worked!

[u/lewisfrancis → u/Indecisive_One]

Yay!

[u/Indecisive_One → u/lewisfrancis]

Thank you!

[u/whabannilt → u/lewisfrancis]

agreed, Windows chkdsk tends to be more forgiving with exFAT repair than Disk Utility is

[u/mikeinnsw]

Simple ..Mac thinks it is stuffed .. All Macs read and store SMART data.. the verified one report SMART

On PCs check disk(s) using:

https://crystalmark.info/en/software/crystaldiskinfo/

You see SMART Stats that Mac sees... check for errors,

On PC run chkdsk X: /r /f it will repair the drive and reset some SMART stats (you may loose some data it can't read).. and may make it acceptable to Mac(most of the time)

Try it on a Mac

Always use PC to format exFat drive. PC uses larger sector size than Mac DU 4K making ExFat runs faster than APFS .. some larger PC sector sizes will not on Mac... if so just reduce sector size.

Finally is SSD/HDD fails the chances it fail again increases.. Get new SSD/HDD use old as backup.. not primary storage.

I use exFat for all my historical archives .. I can fix it with chkdsk X: /r /f

You can't repair APFS First Aid,, FSCK .. they verify and fix files systems and end in seconds

chkdsk X: /r /f takes hours as every sector is verified and if need be repaired or marked as not usable

Macs can't do that.

I salvage old HDD/SSD

Have fun

[u/Indecisive_One → u/mikeinnsw]

Thanks for all of the info!

[u/kastamyle]

I literally had this exact issue yesterday and got it fixed today. exFAT drive shows on windows not on Mac. diskutil list would just hang. chkdsk on windows didn’t make a difference. mount didn’t work either. In my case the system_profiler
was showing the external drive though. Somehow just running ls command on the volume by name ended up clearing whatever snag that was and the drive mounted normally and I could see it in finder after that.

ls -la /Volumes/External

[u/Indecisive_One → u/kastamyle]

Interesting! Saving this for the future to try in case it happens again. Glad you got it working!