Is it possible or likely for malware or a virus to be transferred by plugging a USB hard drive into my Mac? I am in the process of transferring data from my old MacBook Pro to a new MacBook Pro. In order to transfer some files, I decided to plug in a USB hard drive that contained a bootable partition of my old MacBook Pro, as well as a partition that contained the files that I wished to transfer. (The drive contained a few partitions, all of which mounted on the new MacBook Pro).

I had my old MacBook Pro for about four years, and I never performed a fresh install. So since the bootable partition on the usb drive is an exact copy of my old MacBook Pro hard drive, if there were any malicious files, they would be sitting on the hard drive.

When I plugged in the hard drive to my new Mac, I did not open or run any files, I simply copied over a few specific files including some pictures that I took. I did not boot from the partition on the drive.

My understanding is that since macOS does not have autorun, I would have to actually copy infected files, or open or run an infected file (by clicking on it) in order to transfer or 'activate' malware or a virus, is this true?

So assuming that I did not copy or run any infected files from the hard drive, do I have anything to worry about? Is my new Mac at risk? Would it be safe to continue this practice of transferring files in the future?

Also, my new Mac is running macOS Sierra.

Note: I do not have an specific reason to believe that the old Mac is infected - it hasn't necessarily been behaving strangely, I am just cautious since it is an older machine that has four years worth of software installed.
Possible: Yes.
Everything is possible. You may have an infection as yet undiscovered which can make the leap, or one buried in some seemingly innocuous file you're about to copy over.

Likely: No.
With no autorun, it is highly unlikely anything can make the leap of its own accord.

Mitigation...
Get an antivirus, there are plenty about; well-known from the Windows world.
Malwarebytes is still free, &amp; there are free versions of some of the others.  

AV-TEST.org is one of the closest things to an actual authority on the subject. They ran some tests on Mac antivirus apps recently &amp; their findings are in that link. Be aware that some of the slower performers really do give a noticeable speed impact even on my 12-core Mac Pro.
Malwarebytes isn't on the list, but tends to be regarded as "something to have as well as an antivirus".

No protection is ever going to be absolutely 100% guaranteed perfect, but just because "everybody" says Macs can't catch viruses doesn't mean they're actually right, &amp; you're always better safe than sorry.