The drive shows up, you can open your files, but you can't copy anything onto it, rename or delete. One of the most common questions after moving to a Mac — and the first step is always the same: find out what format the drive has, and whether macOS mounted it read-only.
Step 1: check the drive's format
- In Finder: select the drive in the sidebar and choose File ▸ Get Info (⌘+I). Format shows the file system. If Sharing & Permissions says "You can only read", macOS mounted it read-only.
- In Disk Utility (Applications ▸ Utilities): select the drive to see its format and capacity.
- In CrossDriver Files: every drive under This Mac shows its format — APFS, HFS+, exFAT, FAT, NTFS, SMB — and "Read-only" right in the navigation pane. Home adds capacity bars and free space.
Step 2: what the format tells you
| Format | On a Mac | On Windows | Good to know |
|---|---|---|---|
| NTFS | Read only (by default) | Read and write | The usual format of Windows drives |
| exFAT | Read and write | Read and write | Good for drives used on both; no 4 GB file limit |
| FAT32 (MS-DOS FAT) | Read and write | Read and write | Can't hold a single file larger than 4 GB |
| APFS, Mac OS Extended (HFS+) | Read and write | Not without extra software | Mac formats |
If the drive is NTFS
macOS reads NTFS drives but can't write to them by default. Two ways out:
- Reformat it as exFAT if the drive can be emptied: copy everything off, then in Disk Utility select the drive, click Erase and choose ExFAT as the format. Copy your files back afterwards. Erasing deletes everything on the drive, so make sure your copy is complete first.
- Install an NTFS driver if the drive has to stay NTFS — for example, because it's full of data and shared with Windows PCs. NTFS drivers for Mac, such as our own CrossDriver NTFS for Mac, add write support.
If the format is writable but the drive is still read-only
Read-only doesn't always mean NTFS. If an exFAT, FAT or Mac-formatted drive is read-only:
- Write-protect switch: some SD cards and USB sticks have a small lock switch. Slide it to unlocked and connect the drive again.
- Disk errors: macOS can mount a drive with file-system errors read-only. Run First Aid on it in Disk Utility. A drive that was unplugged without ejecting, on a Mac or on Windows, is a common cause.
- Permissions: in Get Info, check Sharing & Permissions. For a Mac-formatted external drive, selecting Ignore ownership on this volume often helps.
Can't copy a large file to a USB stick?
If the drive is writable but a big file (a video, a disk image) won't copy, the drive is probably FAT32, which can't hold a single file larger than 4 GB. Reformat it as exFAT as described above, or compress the file into a ZIP — CrossDriver Files uses the standard ZIP64 format above 4 GB, so the archive opens on Windows too.
Check before you copy
Checking the format before you start avoids many "won't copy" and "won't open on the other computer" surprises. In CrossDriver Files you see it without opening anything: format and "Read-only" are next to each drive in the navigation pane. CrossDriver Files shows only what macOS reports, and works completely without an NTFS driver.