You always make backups to external drives. If you backup to the same drive you use to run the computer and that drive fails, then you don't have a backup.
Time Machine is an adequate backup utility but it requires a lot of storage space - at least two or three times the capacity of the drive you are backing up. For a 512GB SSD, you would want a 1TB or 2TB backup drive, for example. However, Time Machine is not enough. You should have a second backup just in case. This backup should be a bootable clone of your internal drive. You can make a bootable clone using an inexpensive 2.5" HDD or SSD. You can create a clone many ways, but I suggest using Carbon Copy Cloner or SuperDuper! which are relatively inexpensive backup utilities.
Basic Backup
For some people, Time Machine will be more than adequate. Time Machine is part of OS X. There are three components:
- A Time Machine preferences panel as part of System Preferences;
- The Time Machine application located in the Applications folder. It is used to manage backups and to restore backups.
- Time Machine requires a backup drive that is at least 2-3 times the capacity of the drive(s) it backs up.
To use Time Machine you will need a separate backup drive formatted to use HFS+. This can be an HDD because speed is not that important for backups. The backup drive needs to be at least 2-3 times the size of the source drive. For a 250GB SSD, I would recommend a 1TB backup drive. These aren't too expensive. Visit OWC to see what your options are.
Your SSD can be formatted to use APFS, but Time Machine will not work with that format and should be formatted Mac OS Extended, Journaled.
Backups should always be made on a dedicated drive. If you have extra storage needs, then those also should be made on a separate drive. The Startup Drive should not be used for anything more than a startup drive.
Alternatively, get an external drive at least equal in size to the internal hard drive and make (and maintain) a bootable clone/backup. You can make a bootable clone using the Restore option of Disk Utility. You can also make and maintain clones with good backup software. My personal recommendations are (order is not significant):
Visit The XLab FAQs and read the FAQ on backup and restore. Also, read How to Back Up and Restore Your Files. For help with using Time Machine visit Pondini's Time Machine FAQ for help with Time Machine. This is an extensive site with most everything you may need for using Time Machine. You will not find a better resource for Time Machine in one location. Due to the passing of James Pondini, the site is no longer being updated. Therefore, some of the information may be a little outdated.
Although you can buy a complete external drive system, you can also put one together if you are so inclined. It's relatively easy and only requires a Phillips head screwdriver (typically.) You can purchase hard drives separately. This gives you an opportunity to shop for the best prices on a hard drive of your choice. Reliable brands include Seagate, Hitachi, Western Digital, Toshiba, and Fujitsu. You can find reviews and benchmarks on many drives at Storage Review.
Enclosures for FireWire and USB are readily available. You can find only FireWire enclosures, only USB enclosures, and enclosures that feature multiple ports. I would stress getting enclosures that use the Oxford chipsets especially for Firewire drives (911, 921, 922, for example.) You can find enclosures at places such as:
- Cool Drives
- OWC
- WiebeTech
- Firewire Direct
- California Drives
- NewEgg
All you need do is remove a case cover, mount the hard drive in the enclosure and connect the cables, then re-attach the case cover. Usually, the only tool required is a small or medium Phillips screwdriver.
