# One Mac per disk

> Why an iSCSI disk can only be used by one Mac at a time, how BeamDisk enforces it with persistent reservations, how to take a disk back from another computer, and how to move a disk to another Mac.

A file system such as APFS expects to be the only one writing to its disk. If two computers use the same iSCSI disk at once, they overwrite each other's changes and the disk is corrupted.

BeamDisk prevents that. While a disk is attached, BeamDisk has **exclusive access** to it on the server, using SCSI persistent reservations. Another computer that tries to read or write the disk is refused, and BeamDisk keeps the reservation through reconnects and iSCSI service restarts. If the server drops it anyway, BeamDisk takes it back, and the disk's **Last event** says so.

## What you see

- **Reserved by another computer**: **Another computer is using this disk. It connects here once the other computer lets go of it.** BeamDisk keeps trying in the meantime. A disk you ejected here can read this too, once another computer has started using it: attach it again when that computer lets go.
- **Reservation lost**: another computer took the disk while this Mac had it, so reads and writes to it fail here. You get a notification, **Another computer took Projects**, and the menu-bar item shows a red dot.

## Take a disk back

Only do this if the other computer is off, or has disconnected from the disk. If both write to it, it will be corrupted.

1. On the disk's page, click **Take Over…** (it's also in the Overview, in the alert for a lost reservation and when you right-click the disk in the table).
2. Read the warning. If this Mac still has the disk attached, BeamDisk ejects it first, and changes not yet written to it are lost: the server already refuses this Mac's writes.
3. Type the disk's name to confirm, then click **Take Over**. BeamDisk connects the disk and attaches it again.

## Move a disk to another Mac

1. On the Mac that has the disk, quit the apps using it, eject it, and choose **Remove…** from its Options menu. The disk and its data stay on the server; this Mac just stops connecting to it.
2. On the other Mac, allow its initiator name on the target, then add the disk with **Add Disk**.

**Eject** lets go of the disk too: another computer can use it until this Mac attaches it again. **Disconnect All** lets go of every disk, until this Mac connects again.

> **Your target needs persistent reservations**
>
> TrueNAS supports them. A target without them still connects, but without this protection.