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Bluetooth crisis
Q Whenever I use my new MacBook Pro near my iMac, the latter’s Bluetooth starts to play up. At first, it loses its wireless keyboard and Magic Trackpad 2, which then reconnect. It often worsens to the point where I have to plug them both in, using their charging leads. The iMac is running macOS 10.12.6 and uses an Ethernet connection, with Wi-Fi turned off. The MacBook Pro runs 10.13.4, connects over Wi-Fi, and carries on working fine. Why?
AWhat’s most likely to be happening is that the MacBook Pro is trying to talk to your iMac due to their close proximity. That behaviour is extensively employed by the Continuity set of features; these are intended to make it easier for you to use multiple Apple devices together – for example, Handoff for moving a task from your iPhone to your Mac (or vice versa), and AirDrop for sending files or other info directly between devices, even when there’s no local area network available to facilitate this.
These features use Bluetooth to detect the presence of nearby devices, and establish a direct connection between them using Wi-Fi. As your iMac has its Wi-Fi turned off, this can’t function fully, causing the connection to drop, then start all over again.
This combines with a long-standing problem with some Macs running El Capitan or Sierra, whose Bluetooth has never been entirely stable: many people still suffer from sporadic spontaneous disconnect-reconnect cycles. When that wobbly system is then bombarded with proximity-based attempts to connect, it can be sufficient to break its Bluetooth service altogether. That seems most likely when the other Mac is a recent MacBook Pro updated to 10.13.4.
Hopefully, Apple will fix this in a future update. Until then, you have a choice between turning off Bluetooth on the MacBook Pro, preventing use of features that use proximity, or turning on Wi-Fi on the iMac, which may allow the two computers to connect fully for Continuity features and for everything to settle down again.