"An unknown error occurred (32)."

I encountered the above error when trying to restore my iPhone, and searching the Community here yielded a number of posts about error 32 coming up, and the ones I reviewed either ending unresolved or with an answer linking to the iOS update and restore errors page, which doesn't actually list error 32. I'm posting this as an additional troubleshooting step because it worked for me when all other suggested steps did not, and because the posts I found about error 32 are no longer open for comments.

The error 32 may be caused by a malfunctioning USB port or ports. I tried plugging my iPhone directly into all four USB ports in sequence, and none of them allowed the iPhone to appear in the Finder unless I put the iPhone into DFU mode. I tried plugging it into a four-port hub connected to my iMac, which caused the iPhone to appear in the Finder. So there was an issue with the USB ports on the iMac. After resetting the SMC, I plugged the iPhone directly into one of the iMac's USB ports and it finally appeared in the Finder without requiring DFU mode. Restoring and updating then worked.

Long story short: error 32 may appear when the USB ports on your computer aren't working correctly, so resetting the SMC may fix the issue.