Big Sur Disconnecting My External HDD

I am having this problem too. It is happening on a new MacBook Pro, M1 chip, purchased in December, running 11.1. I have two other MacBook Pros, the first running 10.13.6 (late 2011 model), and the second also running 11.1, purchased in June 2020 (Intel chip), and the problem does not occur with either of these computers, with the same external drives. I wonder if it is a hardware issue? I'll be putting a call into Apple when I have time, I'll report anything useful back.

Exactly the same experience and solution I tried. My drive is an older OWC 60Gb SSD using SATA to USB cable. Does not disconnect during drive activity, just randomly throughout the day with an overabundance of annoying notifications that I can't seem to exclude.

I've had the same issues and got so frustrated that I had to go back to Catalina... I could not afford to lose my data after my hard drive kept ejecting and possibly corrupting each time. They need to fix this asap... I'm staying on Catalina until that is corrected.

I continue to have this issue no matter what I have done to avoid it. I am going to try adjusting my settings to never let the computer go to sleep, I am running the M1 Mac mini, and will report back in a few days. The amount of times I come back into my office to find my external HDD enclosure powered off is unacceptable. If this doesn't work, I'll resort to contacting Apple directly, though I am unsure what that will solve, as the issue is random and not user repeatable...

An additional observational datapoint, this occurs with me with the M1 mini and I found out how to reproduce (and work around). For me, this only happens when using Adobe Premiere CC using Rosetta 2 "intel emulation."

I've found that whenever the app writes to a thunderbolt drive (standard USB works fine) it will immediately disconnect and reconnect. My steps to repeat were simple: using adobe premiere CC if you "save" a snapshot of the current timeline frame (export as PNG for example) to the remote thunderbolt disk, it disconnects (never writes) then reconnects again (giving the warning notifications). Every. Single. Time.

When rendering, the minute adobe premiere writes to the 'cache' (a separate external thunderbolt disk) I get the notification again and again for each write operation.

I am not sure if this is a rosetta 2 thing or an adobe issue, but I'd imagine if you have adobe products open in the background, if they do write operations on that disk, you'll see a flurry of notifications.

Since the OP is apparently on an intel Mac and so am I (suffering the same problem), it is unlikely to be rosetta. I also do not run any Adobe products and still have the issue. You might want to make a new posting that makes the rosetta and adobe parts prominent. It might get the issue more attention from others suffering the same.

I ran without DISK NOT EJECTED PROPERLY from Jan 9th to Jan 21st.

I disabled "Prevent computer from sleeping automatically when the display is off"

and the disk EJECTED during the first automatic sleep.

There may be other factors, but for me, the HDD used for TimeMachine, will spontaneously NOT EJECTED PROPERLY when the machine goes to sleep.

This is crazy. I put a USB-powered Seagate HDD on my Mac Mini and set it up as my Time Machine disk. All was good for a few months and then it started exhibiting this problem. Based on 'whatever' I replaced that with an externally powered USB drive (8TB Seagate), and it, too, worked fine for about a month, and now this:

(typical -- starting about a week ago)

I have tried Disk First Aid (repeatedly) and reformatting the drive, and yet the problem continues.

My Mac Mini is running fully updated Big Sur, and the drive is directly connected to the Mini (not through a hub or anything).

Since it can occur so frequently, I can see (in Time Machine) that the drive does appear to disconnect/reconnect, but I'm eventually getting 'good' backups.

What is the resolution?

So for 3 days now, I have had no issues with my external HDD disconnecting, what I have done is set my computer to never sleep. I set the slider for ‘Put monitor to sleep after’ to never, and it seems to have solved the problem. Of course now I have to manually turn my monitor on and off, which is less than convenient, but at least now I don’t have to worry about any possible data corruption...

DISK NOT EJECTED PROPERLY - when Energy Saver setting not to sleep. :(

With Energy Saver settings: x o x o x (no sleep) I ran Jan 9-21 without an EJECT.

I then enabled sleep and it EJECTED on the first sleep.

I set Energy Saver settings: x o x o x (no sleep) and ran 5 days with a EJECT.

However, I did have en EJECT last night with sleep disabled. :( Disabling Sleep appeared to fix the problem until now.

Going to restart with sleep disabled and see what happens.

I just posted a similar topic, adding that when the external HDD disconnect happens, Bluetooth is disabled, peripherals disconnect, and the iMac doesn't respond even after connecting a wired keyboard and mouse. I have two external HDDs, each with separate TimeMachine backups, and separate volumes for data. Only one of the HDDs presents this problem, ejecting the TWO volumes (data and TM).

I have some considerable progress -- Your mileage may vary, but:

As I noted earlier: I had swapped external USB 3-connected hard disks, and the problem remained: Initially all was reliable, but 'soon' (weeks) both disks (one USB-powered, one externally-powered) started suffering this problem -- as frequently as several times a minute. I'm running a late-model Mac Mini with Big Sur, fully updated, and NO sleep nor power-saving enabled, and the disks were only being used for Time Machine.

Desperate for some fix, I bought a new USB Type-C to Micro-B 3.1 Gen2 cable (the latter is the funky connector used on my external disks), and connected it to the very same 'failing' externally-powered disk.

Three days later and NO spontaneous disconnections! (so far...)

I don't know if the difference is that I'm using a USB-C port (instead of the USB -3 Type A port) on the Mac Mini, or if I had a questionable USB cable (I had changed neither when I swapped disks).

The above SEEM to implicate either a bad/questionable cable, or the USB-3 Type-A port on the Mac Mini. Frankly I sorta don't care, as it seems to be solid now(!)

I have a Mac mini 2018 and 3 external hard drives connected. All of them are encrypted. Connection of 2 of them is over USB-C to USB-2 adapter and the third one directly from a USB-2 outlet. I have no problem with that, and the computer can go to sleep without the drives disconnecting. One of them is not a time machine.

To those who have still have the problem, does the problem also occur with a simple USB stick?

Yep, you got the same issues as many of us are having, I have found it's often easier to just reboot than to try to clear all the messages. Also, if you just let this go on for a day or so, I've found Finder is hung and you have to reboot to get the machine to respond without being super sluggish waiting on I/O operations (e.g. use the "Open.." option on apps and attempt to open a file)

As I pointed out earlier: I WAS plagued by this problem on two different HDDs (up to several times per minute!), a bus-powered and an externally-powered, and I verified that Sleep was not involved.

THEN I changed the USB cable (both drives had used the same USB Type-A cable that came with the first drive). I replaced it with a USB-C cable.

It has been three weeks and I have not had the problem At All. I leave my Mac Mini on 24/7.

You may have different circumstances, but this is mine.