External Storage

Captua can save captured images to an external USB drive or SD card instead of the unit’s internal storage. This is useful when digitizing large collections that would exceed the capacity of the built-in microSD card.

Experimental feature

This feature depends on operating-system-level disk operations. Compatibility varies by drive format and hardware. See Supported formats below. Do not rely on an external drive as your only copy of digitized material — always keep a backup.

How it works

While an external drive is active, Captua saves every new capture to a dtk-projects/ folder on that drive. This includes new captures for volumes you started on the internal card, so the images of such a volume end up on two disks. Images captured earlier stay where they are and remain readable. To keep each volume on one disk, activate the drive or restore internal storage between volumes, not while a volume is in progress. Only one external location can be active at a time.

The note under Storage drive in Settings, “Only new projects will be saved to the selected drive”, is out of date and will be corrected (NEH-325). This page describes what Captua does.

If the active drive is disconnected or not mounted, Captua does not switch to the internal card. It refuses to save new captures until an administrator connects and activates the drive again, or restores internal storage.

Preparing the drive

Recommended format: exFAT

exFAT is the recommended format for external drives used with Captua. It is:

Compatible with Linux, Windows, and macOS (no drivers needed on any platform)
Supports files larger than 4 GB (important for high-resolution image sets)
Well-supported on Raspberry Pi OS

How to format a drive as exFAT

Linux / Raspberry Pi

# Install the exFAT tools if not already present
sudo apt install exfatprogs

# Identify the drive (look for your USB device — e.g. /dev/sda2)
lsblk

# Format the partition (replace /dev/sdXN with your actual partition)
sudo mkfs.exfat /dev/sdXN

Caution

Double-check the device path with lsblk before running mkfs. Formatting the wrong partition will permanently erase its contents.

Windows

Open File Explorer and right-click the drive → Format…
Set File system to exFAT
Click Start

macOS

Open Disk Utility (Applications → Utilities → Disk Utility)
Select the drive in the left panel
Click Erase, set Format to ExFAT, then click Erase

Format compatibility summary

Format	Works with Captua	Notes
exFAT	✅ Recommended	Best cross-platform choice
ext4	✅ Supported	Linux-only; slightly faster
FAT32 (vfat)	✅ Supported	4 GB file size limit
NTFS	⚠️ May work	Requires `ntfs-3g`; not guaranteed
APFS	❌ Not supported	macOS-only filesystem
HFS+	❌ Not supported	macOS-only filesystem

Using external storage in the UI

Note

Only an administrator can do this. Settings does not appear in the sidebar for operators and reviewers.

Go to Settings (the gear icon in the sidebar). In the Storage section, click Storage drive (marked experimental) to expand it.

Step 1 — Connect the drive

Plug the USB drive or SD card into the Raspberry Pi. The drive does not need to be mounted beforehand.

Step 2 — Mount the drive

The device list shows the partitions Captua can use. Unmounted partitions have a Mount button.

Find the partition you want to use in the list
Click Mount
Captua mounts the drive, and the row then shows Activate and Unmount

Step 3 — Activate the drive

Click Activate next to the mounted partition. Captua:

Creates a dtk-projects/ directory on the drive (if it does not exist)
Sets that directory as the active projects location
Shows a green active badge on the device row, ✓ In use in place of the Activate button, and a banner confirming the active path

From this point, every new capture is stored on the external drive.

Deactivating / reverting to internal storage

Click Restore default in the banner to switch back to the internal storage. The external drive is not unmounted — existing projects on it remain intact and accessible.

Safely removing the drive

Before physically unplugging the drive:

If it is the active storage location, you can click Restore default first. Clicking Unmount on the active drive also switches back to internal storage.
Click Unmount next to the partition
Wait for the success message
Unplug the drive

Captua can unmount only drives it mounted itself with Mount.

Important

Always click Unmount before unplugging. Removing a drive while it is mounted can corrupt the filesystem and cause data loss.

Troubleshooting

Drive not appearing in the list: Captua lists the partitions it can use. It leaves out the operating system’s own partitions and partitions whose filesystem it cannot use, such as APFS and HFS+. A drive formatted without a partition table does not appear either: create a partition on it with Disk Utility or another partitioning tool, then format that partition (for example /dev/sdX1) as exFAT. If the drive uses APFS or HFS+, reformat it as exFAT as well. Then click Refresh list. If the drive still does not appear, check that it is properly connected and recognised by the OS (lsblk in a terminal).
“Mount” returns an error: The filesystem may have errors, or the drive may need additional kernel modules (NTFS). Reformat the drive as exFAT and try again.
“Activate” fails with a permission error: This can happen on freshly formatted ext4 drives where the filesystem root is owned by root. Captua attempts a fallback using elevated privileges. If it still fails, reformat the drive as exFAT.
Drive mounted but read-only: Some drives enforce read-only mode when almost full or when the filesystem has errors. Check available space and run sudo fsck /dev/sdXN (with the drive unmounted) to repair any filesystem errors.