which best flash drive to use on mac

Flash drives aren’t usually used with Time Machine, USB 3.0 hard disk drives (HDDs) are typical, or USB 3.0 SSDs if you really want to spend a whole lot.

HDDs have high capacity and low price, and with decent longevity. They’re a reasonable compromise.

Flash drive capacity is low, performance isn’t all that great, they’re not good at lots of writes, and longevity tends to be spotty.

If you have a MacBook Pro and have the budget for it, a Network Attached Storage (NAS) server with Time Machine support can be configured for backups whenever the Mac is connected to the same Wi-Fi, and without requiring cabling.

As for capacity, probably double or triple the capacity of the aggregate input storage. More gives you deeper backups. Lately, 4 to 6 TB HDDs have been around the best compromise, in terms of price and capacity. Though pricing can vary.

For various installations, I'm working with Synology NAS servers, each with an array of NAS-rated drives.

Synology has a list of the tested and supported HDD drives for their NAS boxes.

For the most recent installations of those, 12 TB NAS-rated HDDs tended to be the best compromise, but I haven't checked big HDD prices lately.

Whatever NAS you do pick for this, you will want one that provides Time Machine support. Not all offer that feature.

NAS can get as big and as expensive as you need or want or can afford.

For a small configuration, a two-bay (RAID-1 mirrored) Synology DS220+ might interest. That's usually a step or three up from what a Time Capsule once provided. If you decide to add RAID, there are RAID calculators around the 'net, where you can enter the RAID level—and I'd avoid using RAID-5—and the number of HDDs and the size of the HDDs, and the calculator will tell you how much available storage you have with the configuration.