"How do I back up MacBook Air HDD without waiting for iCloud Drive sync?: Sequoia 15.3.1. Cannot backup HDD because ...some iCloud Drive files have not finished syncing. How to accelerate the syncing? How to back up the laptop HDD without having to sync? Purchased new MacBook Air recently."
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Backing Up Lagging:
Are you Using Wi-Fi here? Is this a Public LAN or is it a Home LAN? On a Public LAN, if your network traffic is too high, then expect to be backed down, eventually leading to being kicked off of the network. As for a Home LAN, upgrade your Internet speed from your ISP, and get a router and modern that is cable of keeping up with these specs. Data Transfer Rate would overrule here. So, try using Ethernet. Once connected, turn off Wi-Fi. If you have no Ethernet port on your Mac, then you'd need this adapter: Belkin USB-C to 2.5Gb Ethernet Adapter - Apple