"I have a Samsung external Blu-ray writer that used to work fine with the supplied single cable. But I rarely use it and something must have changed because now it doesn’t have enough power to read Blu-rays."
How many OTHER devices do you have plugged into your USB ports?
The Mac has "only so much" USB power it can supply.
If you have other devices connected that are also consuming power, the Mac may not "have enough left" to run the Samsung drive.
What I'd try:
1. Disconnect all other un-necessary USB devices.
2. Do an SMC reset
3. Now try the Samsung drive again.