Photos app on Mac Mini M4 slow with external SSD

smithrog34 wrote:

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A SanDisk Extreme has been plugged in and set as the system library, and the beach ball has been spinning for 30 mins. My library is about 3.2TB on a 4TB drive.

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Your drive seems to be too small for such a large Photos Library.

After moving the library to a new drive it will need to be rebuild and analyzed again. Rebuilding a library will require lots of free storage on the drive with the library. Try to keep at least enough free storage on your external drive to be able to hold 50% of the size of the library.

Also - if the Photos Library is so large and the Photos Library is already in iCloud, I would not move it to a new external drive, but start over with a new, empty library and just let it download from iCloud to the new drive. After moving an iCloud library to a new drive, Photos will need to sync it again with iCloud and compare the complete library to the existing library in iCloud. This can take weeks before Photos will be back to normal. You will avoid the lengthy new upload and merging of the moved library into the existing iCloud Library, if you start over with a new empty library.Then you will just have to wait for the download from iCloud to the new library location. It will be much faster.

If you haven't already, you should check cables, ports, and plugs. USBc cables all seem to look alike, but they can have vastly different data speeds. On a whim I bought a USBc Cable checker on Amazon for $15

which is kind of fun. (There seem to be lots of different ones.) It's more complicated than it needs to be, maybe because I bought the cheapest one?-- it gives results for each of 18 wires in each direction, 36 lights to interpret, when I'd like it just say "good" or "bad". But It is giving me a hint as to which cables I should throw away.