Can’t upload pictures from iPhone to iMac external hard drive help!

Super strange I bought an external hard drive to keep my photos on as I have quite a few. Worked totally fine for a long time. I kind of blew it and let my pictures add up a lot and had like 14,000 to upload… I uploaded like 6000 and then it stopped and now everytime I connect the phone it shows the rest and when I click upload nothing happens Using a Lacie d2 professional 8tb external hard drive always connect to m1 iMac with 1 tb storage. Anyone able to help me out?