Unmounting external drives - is it really necessary?

[canhaz]

I dock my MBP multiple times a day with a single TB3 cable. The TB3 dock has 2 APFS NVME drives on auto-mount (for backups).

Apple tells me to do it.

My question is how necessary is this really?

It's a bit of a pain, so if it's not going to corrupt anything practically, that's of course a preferable route to a manual action needed every time. OTOH if the backups are going to be corrupted, obviously it needs to ejected then.

IOW does anyone have any data or anecdote about how disconnecting external drives is affected by HFS+ vs APFS or HDD vs SSD vs NVME or Thunderbolt vs USB?

Because currently the warning seems to paint these multiples scenarios under the same brush of external disks.

If anyone has a story or data on this other than "Just unmount them, better to be safe than sorry" then feel free to chime in.

 

[Jack Neill]

I dock my MBP multiple times a day with a single TB3 cable. The TB3 dock has 2 APFS NVME drives on auto-mount (for backups).

Apple tells me to do it.

View attachment 932726

My question is how necessary is this really?

It's a bit of a pain, so if it's not going to corrupt anything practically, that's of course a preferable route to a manual action needed every time. OTOH if the backups are going to be corrupted, obviously it needs to ejected then.

IOW does anyone have any data or anecdote about how disconnecting external drives is affected by HFS+ vs APFS or HDD vs SSD vs NVME or Thunderbolt vs USB?

Because currently the warning seems to paint these multiples scenarios under the same brush of external disks.

If anyone has a story or data on this other than "Just unmount them, better to be safe than sorry" then feel free to chime in.

Ive had issues in the past with drives not mounting correctly if they aren't ejected previously.

 

[casperes1996]

Soooo, short answer is that most of the time you'll be find, but various forms of corruption can occur on any type of drive if you don't unmount; But how are you currently unmounting? Maybe speeding it up for you will make it less bad. If you select the volumes on the Desktop (just like marking them) and click cmd+e it ejects. You can drag to the trash as well to eject volumes, the icon turns to an eject symbol instead of trash. You could also get an "eject all" menu bar item or we could figure out a way of attaching a shortcut key to unmount all

Long answer:
When data is entirely at rest and there are no open file descriptors on the attached volume, forcefully removing it will not have any negative consequences. However, you cannot just rely on you not manually transferring any data. Background processes could be using the external drive, and even tasks you've initiated that look like they are done could not be properly flushed but still sit in volatile memory, which could lead to corruptions. You can force a flush from the Terminal as well with the flush command which means that all data in volatile memory set to be placed in permanent storage once the buffer stream is filled will immediately be saved to disk. As a side effect of this it means that if a drive, in theory, could complete all data transfers instantly, the likelihood of it being corrupted from forceful unattachments is very limited since it'd have to be disconnected in the instant it was initiated. But many drives, SSD or HDD will buffer up transfers meaning that the task will be reported complete by the OS and seemingly be complete before it actually is flushed to permanent storage, so disconnecting at that moment will, no matter the disk type, corrupts something, perhaps even invisibly, so it may not be as extreme as the drive not mounting, but just a single file or whatever in your backup not working.

This is the case on all operating systems btw, but a mitigation that can be made is to force data never to buffer and always flush immediately, which I believe is the default in newer releases of Windows, meaning external drives operate slower but are a bit safer to randomly disconnect, though there can still be issues, though much rarer you don't pull the plug while transferring data.

 

[canhaz]

Thank you for the explanation. Might just map a touchbar key to eject all drives. Seems like workable compromise.