What an operating system does
An operating system (OS) is the software that sits between the hardware and everything else. It does four jobs:
- Starts the computer and controls its hardware (processor, memory, storage, screen).
- Runs applications and shares the hardware between them.
- Stores and retrieves files.
- Gives the user an interface: windows, icons, a touch screen or a command line.
A useful comparison is a building manager. Tenants (applications) never touch the wiring or plumbing (hardware) directly; they ask the manager.
Identify the workstation OSs
A workstation OS runs on a desktop or laptop. CompTIA lists four:
- Windows (Microsoft): the most common business desktop OS. It runs on PCs from many manufacturers.
- Linux: open source and free to use. It comes in many "distributions" such as Ubuntu and Red Hat Enterprise Linux (RHEL), and is common on servers and technical workstations.
- macOS (Apple): runs only on Apple Mac computers.
- Chrome OS (Google): a lightweight OS built around the Chrome browser and web apps. It runs on Chromebooks, which are common in schools.
Identify the mobile OSs
A mobile OS is designed for touch screens, batteries and cellular radios.
- iOS (Apple): iPhone only.
- iPadOS (Apple): iPad only. It is based on iOS, with added multitasking for the larger screen.
- Android (Google): open source, and used by many manufacturers on phones and tablets.
When a ticket names a device, name its OS first. That tells you which tools, settings and update rules apply.
What a filesystem does
A drive is just a huge run of numbered storage blocks. A filesystem is the index that records which blocks belong to which file, each file's name and folder, and (in some filesystems) who may open it. Formatting a drive writes a new, empty filesystem onto it.
Each OS understands only certain filesystems. That is the root of most compatibility problems.
Match each filesystem to its OS and use
| Filesystem | Spelled out | Native OS | Typical use |
|---|---|---|---|
| NTFS | New Technology File System | Windows | The Windows system drive and internal drives. Supports file permissions, encryption, compression and very large files. |
| ReFS | Resilient File System | Windows | Large data volumes where integrity matters. It detects and repairs corruption. Windows cannot boot from it, and it is not supported on removable media. |
| FAT32 | File Allocation Table 32 | Any | Small USB drives and memory cards. Almost every device reads it, but a single file cannot exceed 4 GB and it has no permissions. |
| exFAT | Extensible File Allocation Table | Any | Large removable drives shared between OSs. It removes the 4 GB file limit. |
| ext4 | Fourth extended filesystem | Linux | The default on many distributions, including Ubuntu. |
| XFS | Extended filesystem (CompTIA's wording) | Linux | High-performance filesystem for very large files and volumes. It is the default in RHEL. |
| APFS | Apple File System | macOS, iOS, iPadOS | The default on Macs running macOS 10.13 or later. |
Note: CompTIA's acronym list spells exFAT as "Extended File Allocation Table". Recognize both wordings.
Understand vendor life-cycle limitations
Every OS version has a life cycle set by its vendor.
- End-of-life (EOL): the vendor stops supporting the product. The device still switches on and works, but it receives no more security patches, fixes or technical support. Every flaw discovered after that date stays open.
- Update limitations: a device may be unable to receive a newer version even though it still works.
Examples a technician meets:
- Windows 10 reached end of support on October 14, 2025. Microsoft's Extended Security Updates (ESU) program can supply security updates for a limited time, but no new features and no general technical support.
- Chrome OS devices receive 10 years of updates, counted from the platform's release date, not the purchase date. A Chromebook bought new can therefore be close to its cut-off.
- Android updates depend on the device manufacturer. Two phones of the same age can be on different versions.
- Older iPhones, iPads and Macs are eventually dropped from new iOS, iPadOS and macOS versions.
What to do when you find an EOL system:
- Confirm the OS version and the vendor's support date.
- Check whether the hardware can run a supported version.
- Upgrade or replace it. If neither is possible yet, document the risk and escalate.
Avoid compatibility concerns between operating systems
Common concerns:
- Applications: a program written for one OS does not run on another. A Windows .exe will not run on macOS or Chrome OS.
- Filesystems: macOS can read an NTFS drive but cannot write to it without third-party software. Windows cannot read APFS or ext4 without extra software.
- Hardware and drivers: a printer or scanner may have no driver for a particular OS.
To choose a filesystem for a drive shared between OSs:
- List every OS that must read and write the drive.
- Find the largest single file it must hold.
- If any file is over 4 GB, choose exFAT.
- If all files are under 4 GB and the drive is small, or an older device must read it, FAT32 works.
- If only Windows will use it, choose NTFS.
Example: a video editor moves 20 GB files between a Windows PC and a Mac. NTFS fails because the Mac cannot write to it. APFS fails because Windows cannot read it. FAT32 fails on file size. The answer is exFAT.
What the exam asks about this
Expect short scenarios that ask you to:
- Match a filesystem acronym to its full name and its OS.
- Pick the filesystem for a shared or removable drive, usually with a file-size clue.
- Identify which OS runs on a named device.
- Recognize the risk of an EOL OS and choose the next step.
Read the scenario for two clues: which OSs are involved, and how big the files are.
Key takeaways
- Workstation OSs: Windows, Linux, macOS, Chrome OS. Mobile OSs: iOS (iPhone), iPadOS (iPad), Android.
- NTFS and ReFS belong to Windows, ext4 and XFS to Linux, APFS to Apple.
- FAT32 and exFAT work across OSs. FAT32 limits a file to 4 GB; exFAT does not.
- ReFS is for data resilience, not for boot drives or removable media.
- EOL means no more security patches or support. The device still runs, but its risk grows.
- For a drive shared between Windows and macOS with large files, choose exFAT.