With macOS’ Finder, I was trying to find a specific decade old file in the old back up since 2025! It was connected to an 13" Intel MBP with its macOS Sequoia v15.7.5. HDD is connected through Apple’s USB-C Digital AV Multiport Adapter. Searching was slow in 2 5 TB HDDs with almost 100 GB free. :(
Thank you for reading and hopefully answering soon. :)
If it’s searching at USB 2.0 speed— 480 Mbps or 60 MB/s —then searching over 100 Gb might well take a loooooong time for Spotlight to perform a search. I say this because even though you said USB3, does the digital AV adapter slow things down? Especially if you have just connected it and Spotlight had never previously indexed your volume. I might just let it search for a week!
If you need the file sooner, maybe take your drive to an office store, a Best Buy, a UPS store or a FedEx Kinkos. Even a library if it has newer computers. Maybe that would search more speedily..
Or there’s corruption in the drive and it can’t find stuff because its directory is damaged. One could then try Disk Utility on it to REPAIR the drive’s file structure before searching again.