Photos is Empty But Still Using 30GB of Disk Space

On macOS 11, the "Photos" item in "Manage Storage" is showing you the size of the content of your System Photo Library, it does not tell you the total of all all photos on your Mac. See:

See used and available storage space on your Mac - Apple Support

• For the "Photos" item ist says: "Photos: Contains your System Photo Library (see the Apple Support article Designate a System Photo Library in Photos)."

So you may be seeing the content of your system photos library listed, even if it is not on your system volume, and only the storage used by the media files inside, not the storage used for auxiliary files in the Photos Library, which may be even larger.

You wrote: Taken on notice. I'll look at an alternative. I've been wanting to setup network attached storage... but the SD card was significantly cheaper.

Putting your Photos Library on a NAS will be even more risky and may not work at all. The only supported method of moving your Photos Library to an external storage is an external drive, with a wired connection.If it is the system Photos Library, you have to plug in the external drive and keep it connected as long as you are logged into your user account, as the system library needs to be available for the background processes. Prepare the external volume strictly as described in the linked article, that Tony posted: https://support.apple.com/en-gb/HT201517

Finding out the storage used by image files on the system volume is a bit complicated, particularly, if we are using iCloud Photos and "Optimise Storage". For example, on my Mac with the smallest internal SSD I am seeing this:

• System Information > Manage Storage : Photos : Photos contains photos and Videos using 8.92 GB of Storage.
• The Finder is showing 29.47 GB as the size of the System Photos Library in the Pictures folder.
• When looking at the storage used by photos in Photos, I am seeing in the Info for the Library item: 5.093 Photos, 348 Videos,172.47 GB

I suggest, you should just look at the size of your Pictures folder, to see how much storage the photos are using on your Mac, if you are only keeping all photo libraries in your Pictures folder.

Right, it is not intuitive and confusing, that Photos is showing us storage used in volumes, that are not attached.

But per definition, Photos 6 is only showing the storage used for photos in the system Photos Library - wherever it is stored, even if the drive is off-line. I tested this by copying additional libraries into my Pictures folder, and the storage shown for Photos did in the "Manage Storage" did not change at all.

The huge amount of "Other" is mainly storage used temporarily by the system for caches, log files, etc. This storage is managed solely by the system, dynamically, as space permits. On my other Macs with plenty of free storage the "other" has a size of 400GB, on my smallest Mac it is "only" showing 80GB. When I am using more storage for documents and data, the size of "Other" will drop automatically. You have read this correctly in the support document - we cannot manage the "Other" ourselves. If there are files that can be purged, usually a restart will help, or restarting in safe mode.

Unrelated, but

a usb stick/SD card is not compatible with storing a photos library - you are risking data loss and loss of your photos.

If you use an external drive, it must be formatted APFS, or MacOS extended, with the ignore ownership checked.

See:

https://support.apple.com/en-gb/HT201517

Have you tried a spotlight search for Photo or iPhoto Library? Could you have an old library lying around? Do you still have an old migrated iPhoto Library there?