# File systems

> Which file systems BeamDisk reads and writes on an iSCSI disk: APFS and Mac OS Extended (Journaled) read-write; exFAT, FAT and NTFS read-only; disks from Windows or Linux with 4 KiB sectors read-only; others left unattached.

Each disk is inspected before it appears in Finder, and what's on it decides how this Mac can use it.

| On the disk | In BeamDisk |
|---|---|
| APFS | Read and write |
| Mac OS Extended (Journaled) | Read and write |
| exFAT, FAT, Mac OS Extended (not journaled) | Read-only |
| NTFS | Read-only: macOS can only read NTFS |
| Nothing yet | **Empty disk**: [format it](https://beamdisk.app/docs/disks/format/) |
| Linux, ZFS, VMware and other file systems macOS can't read | Not attached: **Unreadable here** |

A disk that shows **Unreadable here** stays connected but is never attached, so it can't be erased by mistake. Its Contents row says what BeamDisk found, where it can tell.

Some things are APFS-only: [giving free space back to the server](https://beamdisk.app/docs/disks/free-space/), and [using new space](https://beamdisk.app/docs/disks/grow/) after a disk grows. For a disk you'll use only from Macs, use APFS: [Format a new disk](https://beamdisk.app/docs/disks/format/).

## Sector size

Disks have 512-byte or 4,096-byte (4 KiB) sectors, set on the server. BeamDisk reads and writes both, with one exception:

- **A disk written with 4 KiB sectors by Windows, Linux or another initiator** opens read-only. Its state reads **Read-only**, with **4 KiB sectors** beside it in the Overview, and Add Disk warns about it before you add it.
- **A 4 KiB disk formatted on this Mac** can then only be read with BeamDisk, not by Windows, Linux or other initiators. BeamDisk says so on the disk's page. For a disk that may move between systems, use 512-byte sectors (on TrueNAS, keep the extent's default): see [Set up TrueNAS](https://beamdisk.app/docs/targets/truenas/).

## Disks with problems

If a disk's partition table is damaged, overlaps or runs past the end of the disk, BeamDisk opens it read-only at most, and its page lists what it found when it connected, such as **Its partition table is damaged; the backup copy is intact.** A disk made smaller on the server is one way to get there: see [Make a disk bigger](https://beamdisk.app/docs/disks/grow/).

On a disk with several partitions, any that macOS can only read are mounted read-only, and the others as usual.