External Drive Connected to Thunderbolt2 Port Only Mounts Once

[Gord Webster]

I have an external drive that has both Thunderbolt2 and USB3 connections. For the past few years, I have been using it for my Time Machine and using the TB2 connection. Every few days I would power on the drive, it would mount and my TimeMachine would update. After the update, I un-mount and then power down the drive.
A couple of weeks ago (maybe around the time of the update to 10.15.6?) I noticed the drive would no longer mount using TB2. I tried connecting it by USB3 and it works 100%.
By process of trying a second external (TB2 and USB3) drive on my iMac and this drive on my MacBook Pro (with TB2 and USB ports), every combination (TB2 and USB3) works EXCEPT nothing works in my iMac TB2 ports (both) and will not show either external drive. My assumption is there is an issue with my iMac's Thunderbolt ports. A system report does not show anything connected, even with both drives connected.
In reading a few places, one suggestion is to reset the PRAM. I've done that, and the first time after rebooting all is well, and the drive mounts. When I unmount the drive, and try connecting again, nothing. A restart does not resolve the issue. The only thing that allows me to use the TB2 ports is resetting the PRAM.
I also tried resetting the SMC, without success.
For the time being, I can continue to use Time Machine for my backups using the USB3 connection, but I would rather use TB2.
Any suggestions? Thanks in advance

Gord

iMac 17,1 (Early 2015)
macOS Catalina 10.15.6

 

[Cory Cooper]

Hello and welcome.

I apologize...I thought I had typed a response to this the other day, but it doesn't look like I posted it.

It sounds like you have done the troubleshooting steps I would recommend. The only other that comes to mind is it may be a cable issue? Do you have another TB cable to try...maybe not, since they are fairly expensive?

Also, maybe try disconnecting all peripherals and then try another SMC and NVRAM reset in that order.

C

 

[Gord Webster]

Thanks, Cory. I appreciate your feedback and suggestions. I've tried, again, an SMC and resetting the NVRAM, now with limited success. In that, after each restart, it does connect and TimeMachine runs fine. As soon as I unmount the drive, it will not mount again until the next restart.

As you mention, TB cables are fairly expensive, so I have not gone there, yet. I figure the fact that this happens with both of my external drives, and that my portable drive has a built-in TB2 cable I have tried with two different cables. Also, the fact that it works consistently after a reboot, again leaves me to believe the issue is not the cable, but is software related.

I think I will use the workaround until the next OS update, and see if that happens to resolve this issue.

Gord