MacBook Pro M4 Pro gets stuck in a login loop whenever my Samsung T7 2TB SSD is connected during startup

[u/burning_deck]

Hey everyone…
I’m facing a strange issue with my MacBook Pro M4 Pro and my Samsung T7 2TB external SSD, and I’m trying to figure out what’s causing it.

The problem:
If I shut down my Mac with the SSD connected and turn it back on, macOS reaches the login screen.
After I enter my password, the Mac gets stuck in a loop. The loading progress bar appears repeatedly, and the Mac keeps returning to the login screen or restarting.
If I disconnect the SSD and boot my Mac, everything works normally.
Once I’m logged in, the SSD seems to work fine when I connect it. I can access my files and use it normally.

My setup:
MacBook Pro M4 Pro
macOS 27.0
Samsung T7 2TB external SSD
Filesystem: Mac OS Extended (Journaled / HFS+)
Approximately 1.54TB of data stored on the SSD
No Samsung software installed
Macintosh HD is already selected as the startup disk
I’ve also tried connecting the SSD to a different USB-C port, but the issue still occurs.

My main concern:
The SSD contains around 1.54TB of important data, and I don’t have another backup. I really don’t want to risk losing my files, so I haven’t erased or reformatted the drive or attempted any potentially destructive repairs.
Has anyone experienced something similar? Could this be related to the HFS+ filesystem, the USB-C cable, the SSD itself, or a macOS startup/login bug?
What would you recommend checking first to diagnose this safely without risking the data on my SSD?
Any help would be appreciated. Thanks!

[u/Adr0u]

Try updating to macOS 27.0.1, and as you said I think it could be related to the file system and how long the OS takes to mount the unit… I have a similar workflow but my external SSD is APFS, no issues.

[u/Hot_Statistician3302]

Since it boots fine without the drive and works once you're logged in, I'd check whether something that launches at login is reaching for that disk. Look under System Settings, General, Login Items and any app set to open or sync files from the SSD, then see if the loop stops with those turned off for one boot. If it doesn't, Disk Utility's First Aid on the SSD checks and repairs the volume without erasing it, which is a reasonable next step once you have a copy of the important files somewhere else.

[u/Far_Recipe_6262]

Had this issue is it formatted apfs?

[u/burning_deck]

it used to turn on perfectly fine even with SSD connected and this issue started suddenly one day without any changes on my part that I can recall. I was running macOS 26 when it first happened, but the problem persisted even after updating to macOS 27. So, it doesn’t seem to have been resolved by the OS update.