Can I use a USB 3.0 64GB Flash Drive as a backup disk

The cloud can be used; however, it is also recommended to have a physical backup as well. As for your flash drive, make sure it is good quality - or it won't last very long (all drives will go bad - some sooner and some later). And, make sure it has USB 3 ports or it'll be too slow (forget about backing up wirelessly, it'll take days). So, once you have one, 64 GB is a bit small; if you want to use Time machine (which is already on your Mac), it wants to have a drive that is about three times the size of your internal drive because it keeps older copies of your backups. That is handy if you need something that you just deleted - you can go back and retrieve the older copy of it.

After you plug in your drive, open Disk Utility (Applications > Utilities) and choose the drive. Choose to erase it and format as Mac OS Extended (Journaled) and GUID Partition scheme (to make it bootable if you need it). Once that is done, you can simply launch Time Machine and it'll do everything else.

If you don't want to use Time Machine, you can either drag 'n drop your photos, videos, files, etc. or you can also create a bootable clone using CarbonCopyCloner or SuperDuper.

Only for less than 60GBs of data if you limit it to your files. Otherwise, a backup drive should minimally equal the size of the drive it backs up. For Time Machine, the backup drive should be at least 2-3 times the size of the drive it backs up.

Flash drives may be erased like any other drive. They must also be partitioned and formatted for use on the computer. Flash drives used between Macs and PCs for casual file transfers cannot be used for backups.

iCloud cannot be used for normal backups. It can, however, be used to back up certain types of files coming from specific sources like Photos, camera pics, iPhone data, etc.

I consider a backup to be a reliable repository from which I can rebuild my complete computer system in a short period of time. A 64GB flash drive fails on two accounts: it isn't large enough to hold my entire computer system and a flash drive is inherently less safe than a rotational hard drive or SSD. (They are easily lost, are physically fragile, and can be damaged by static shock.) A cloud backup backup fails because (for most of the world) because Internet access is too slow to quickly rebuild the system and most cloud systems service data, not applications and operating system.

For me, the best backup is a set of external hard drives: 1 containing a clone of my computer's hard drive using a program such as CarbonCopyCloner and updated once a day and 1 containing a complete backup of data, applications, and operating system which is updated hourly. The two provide a clone that I can begin using immediately if I absolutely must get right back to the job and a very recent backup so that at worst I've lost an hour of work.

For even better protection 1 or both of these drives should have counterparts off-site and to be swapped out weekly. After all a backup is no good if the thief takes it when he steals the computer.