So you just bought an external hard drive for backups. Now, with what filesystem should you format it? Ext2? FAT32? No matter which one you choose, there are trade-offs to consider.
You face the same choice whenever you buy a USB thumb drive, but for a backup drive, a lot more is at stake. Those backups have to be there and be reliable when disaster strikes. On the one hand, you need to preserve your data and your metadata, so not just any filesystem will do. But on the other, if you're not at your home base, you need to be able to access it from anywhere, so you can't be too obscure.
Back in February, my streak of never needing to restore from backup came to an end (though my no-hard-drive-failure streak is still running strong at 11 years). Filesystem corruption zapped some work from my laptop while I was on the road. It was not a sizable amount in the grand scheme of things, but inconvenient in that it happened while I was away from home base. Once I returned, I started shopping for a pocket-sized external hard drive to carry around to deal with such occasions in the future.